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over all the humps at about 25 knots, where there is a range of about 200 horse-power. The curve for 90 feet of water is, however, for 5000 horse-power at 26 feet, or about 500 horse-power above the next highest curve. It would therefore pay better, according to these model experiments, to run a 26-knot trial of a destroyer in a depth of 20 feet to to feet rather than at a depth of 90 feet; the saving in power for the given speed due to the use of shallower water would be about 600 to 700 horse-power.

We have been obliged to depart from the text of Mr. Harold Yarrow's paper in order to give the facts contained in his diagram. Limits of space prevent detailed reference to other particulars brought forward by these suggestive trials, but enough has doubtless been said to show their interest and the wide field for further investigation that is afforded by the numerical data now at command. It will be remembered, as Mr. Harold Yarrow pointed out, that the tank experiments were made in the usual way without propellers, and this would doubtless have a considerable influence on the results; but possibly a way may be found, as suggested by Sir William White, to add the propeller, and so bring the tank conditions more nearly akin to actual practice. It will be remembered that the late Mr. Froude proposed to run a propeller, worked by independent mechanism, at a speed of revolutions corresponding to that of actual practice, the model, of course, being towed by the carriage.

Mr. Marriner's investigation of the model results showed that they should, as Mr. Harold Yarrow said, “be accepted with caution." In order to check these tank data progressive trials were made with an actual destroyer on a carefully selected course off the mouth of the Thames. Four tugs were anchored to mark the course, which extended from the East Girdler buoy across the Tongue Sand to a point east of the extreme north-east point of the Tongue Spit. This gave water of depths varying between about 14 to 16 feet over the sand, and 100 to 102 feet in the Queen's Channel. The revolutions of the engines, the fore and aft inclination of the vessel, and the height of the stern wave were noted, indicator diagrams also being taken. The data thus obtained at varying mean speeds were given by means of diagrams thrown on the screen. We have not space to repeat all the records, but will take as an example the run made at a mean speed of about 22 2 knots, the steam pressure being 140 lb. In running from rather less than 50 feet depth into water of 20 feet, the revolutions increased from about 305 to more than 325; the inclination in a length of 20 feet decreased from approximately 5 inches to 4 or 4 inches; and the approximate height of the stern wave fell from 20 inches above the deck level to a little more than 10 inches below the deck level, or about 2 feet 6 inches. Naturally any increase or reduction of resistance to the vessel would be accompanied by increase or reduction in the rate of revolutions of the engine, the steam pressure being constant. Increased height of stern wave and greater inclination are also signs of increased resistance and a greater demand for horse-power.

It would be difficult to deal adequately with Mr. Marriner's paper within anything approaching the space we have at our disposal, and without the diagrams by which it was accompanied. We can only hope, therefore, to give an idea of its scope, and refer our readers to the original in the published Transactions of the institution. The contribution consisted of a discussion, on a scientific basis, of the results contained in the preceding paper, the size of waves made by a vessel in her passage being 2 measure of the power absorbed in their formation. As the height of the stern wave was seen to increase when the resistance of the ship increased abnormally, it was to be assumed that anything which tended to retard the formation of waves would reduce the loss from wave making. The author cited the work of W. Froude and his son, R. E. Froude, of Lord Kelvin, D. W. Taylor, of the United States Navy, and Prof. Horace Lamb. The formula for relation of length to speed of ordinary repeating waves in deep water was set forth, and also the more complicated equation for shallow water.

The equa

tion showing critical depth for speed and critical speed for depth was given, and the conclusion was drawn (supported by Scott Russell's equation for the solitary wave

in canals) that the wave at the critical speed changes from the repeating to something approaching the solitary type."

The author next considered the waves accompanying the vessel. Transverse waves should tend to become longer and longer for the same speed as the depth diminishes until, at the critical depth, these should be of the isolated type. In shallower water, past the critical depth, there would be no transverse wave corresponding to the critical depth. Actually as the waves became longer if they did not lengthen as rapidly as investigation would lead one to suppose they would be travelling faster than their natural speed, and must be dragged by the boat. The increased resistance on approaching humps in the curve supports this view. The isolated wave is non-repeating, and exists only under certain relations of depth to speed. After passing the critical point the transverse wave disappears, being replaced by confused water. The paper was accompanied by diagrams illustrating these points, and it was considered a fair deduction that the waves formed by a ship closely follow the laws of waves in open water. The effect of the vertical sides of a canal in diminishing the loss of energy was pointed out by the author, and the manner in which the restricted width of a tank might have a similar effect was noted. The interference of the bow-wave system on the stern-wave system was discussed, it being shown that the union of the bow and stern waves (the crests coinciding) resulted in a large resultant wave which would carry away a great deal of energy. The velocity of diverging waves is much less than the speed of the vessel, speeds attained up to the present not being high enough for waves to approach the critical speeds for the depths in which vessels usually run. Diverging waves apparently constitute the principal wave-making resistance at speeds beyond the critical combinations of depths and speeds under consideration. The final conclusions of the author were as follows:-" (1) The critical combinations of depth and speed do not depend on the size of the vessel. (2) Of these critical combinations there is, for every vessel, one more serious than the others, and where this worst combination occurs depends largely on the length of the vessel. (3) The depth to be avoided is given by the equation d=V10, and the resistance diminishes in both greater and lesser depths. The further away from this bad depth the better, especially on the deep side.'

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In the course of the discussion which followed the reading of these papers, Mr. A. F. Yarrow, in conformity with a suggestion made by Sir William White, proposed, and Mr. S. W. Barnaby seconded, a resolution that the Admiralty be urged to erect a measured mile, where ample depth might be found, in proximity to Chatham and the Thames. This resolution was put to the meeting by Sir John Durston, who occupied the chair, and was carried unanimously. It was but the logical outcome of the facts brought forward. The measured mile in the Clyde at Skelmorlie has ample depth of water, and is, as Sir William White said, the only satisfactory mile for highspeed trials; a fact which, it is acknowledged, gives the ship-builders of that district a manifest advantage over those of other centres. Mr. Barnaby stated that when a destroyer built by his firm, John I. Thornycroft and Co., was tried on the Skelmorlie mile an increase of speed of knot was obtained over that reached under the best conditions on the Maplin mile, whilst an increase of 3 knots was reached as compared to running on Maplin when the state of the tide was most unfavourable. Sir William White, in a letter to the secretary, read at the meeting, heartily endorsed the suggestion of a deepHe also pointed water measured mile off the Thames.


out, with great regret, that it was necessary for Messrs. Yarrow to go to Germany for their tank experiments, and trusted that the fact might furnish a fresh incentive towards the establishment of a research tank at the National Physical Laboratory.

In the discussion en the technical details of the two papers, besides those mentioned, Mr. J. H. Narbeth, of the Admiralty, Mr. R. Saxton White, Captain Jackson, R.N., Controller of the Navy, Mr. W. H. Whiting, chief constructor, and Prof. Biles took part. Generally it may be said the views expressed by the authors were not disputed, although Sir William White did not quite agree

with Mr. Marriner as to the importance the latter attached to the comparative narrowness of the tank.

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On the Thursday's sitting Mr. Milton's paper on fractures in large steel boiler plates was read and briefly discussed. It gave particulars of the failures, the reasons for which could not be explained, of certain plates, and therefore were, naturally, attributed to "heat treatment. is a term of exceeding comfort to the steel maker and the engineer alike, for the former is able to put the blame on the latter, and the latter to put the blame on the former, as no one can prove where the injudicious heat treatment occurred. The controversy is an old one, dating back, at any rate, to the days of the Livadia's boilers. Mr. Milton's paper is a suggestive contribution, and the facts he records may carry us some way towards a solution of the problem in the more or less distant future.

Mr. William Gray in his paper gave particulars of the performances of certain steamers fitted with Parsons' steam turbines. These were set forth in a table, which, as the author said, "treated the matter from a purely commercial standpoint.' The discussion was largely of the same character.

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of upwards of 223,000 lives per annum. As an exceptionally low rate of infant mortality had been maintained for two successive years, it might be hoped that the warnings uttered as regards infant hygiene, and more particularly infant feeding, were beginning to take effect.

It was, however, during the first half of life that the great fall in the death-rate had taken place. It was a remarkable fact that in men, at all ages from forty-five to seventy-five, there had been a startling rise in the death-rate, and that in women, from fifty-five upwards, it had been practically stationary. At the ages when we should have welcomed a rise in the death-rate, and at which only, in a hygienic Utopia, death ought to occurSome of the eighty-five and upwards-it had fallen. nerve centres went on evolving until middle life, e.g. the hand and arm centres. He had ascertained that among certain classes of operatives in Birmingham the hand and arm centres did not reach their full maturity until about the thirtieth year. Similarly with the weavers of Bradford and the potters of Staffordshire. At about forty-five the productiveness of the manufactory hand generally began to diminish, and after that it contracted in an increasing The hand-failure of our operatives ratio as time went on. after forty-five was premature, and due to excessive wear and tear of the mechanism regulating manual movements. But there were other centres in the brain which, reaching Orators maturity later, retained their power longer. secured their greatest triumphs between forty-five and fiftyfive, and it was with musical expression as with oratory. The best antiseptic against senile decay was an active interest in human affairs, and those kept young longest The natural evolution of our nerve who loved most. not centres was largely interfered with and too often arrested by unfavourable environment and deleterious habits of life or methods of work. It was a good working hypothesis that the natural life of man was one hundred, and that so far as it fell short of that it was "curtailed of fair proportion." Every man, he thought, was entitled to his century, and every woman to a century and a little more.

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The only remaining paper was not on the original programme, but was read at the conversazione which brought the proceedings to a conclusion on the Thursday evening. It was a contribution by Captain R. H. Bacon, R.N., entitled "Notes on the Causes of Accidents to Submarine Boats, and their Salvage.' This paper, in harmony with the circumstances in which it was read, was of a popular nature, and was designed to show to the public at large that undue anxiety as to the safety of submarine boats is warranted by the conditions under which they employed. Dealing with the probability of water entering the hull through the hatch (the cause of four serious accidents through which these boats have foundered), the author pointed out that the fitting of another water-tight hatch at the base of the tower reduced the chance of

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accident in the future. The danger from grounding, he
said, was not very great," whilst the risk of the hull
being crushed by the boat diving to too great depths
argued the failure of the diving rudders, or too much
water ballast. As to explosion through leakage of petrol,
"in a properly designed system leaks should be prac-
tically non-existent. Another possible cause of
plosion is due to hydrogen given off when batteries are
being charged, but as this operation is carried on only
when the boat is opened up for ventilation, no danger
Altogether Captain Bacon's lecture was most re-
assuring, and it is pleasant to learn that his optimistic
views are fully shared by his colleagues in the Service, both
officers and men. The risk of sinking-involuntary sink-
ing-being so small, it is of less consequence that only
over a limited area near shore is it possible to recover a
submarine boat once she has gone to the bottom. For
this reason Captain Bacon considers it inadvisable that
the Royal Navy should have a salvage plant of its own.


THE congress of the Institute of Public Health, which
this year was held in London under the presidency
of the Marquis of Londonderry, attracted a large number
of visitors, and much good work was done in the various
sections which met at the Polytechnic and at King's

In a brief space it is impossible to deal adequately with the valuable discussions and papers read.

Sir James Crichton Browne, F.R.S., in his presidential address to the section of preventive medicine, chose as his subject the prevention of senility. It was, he declared, on the reduction of the death-rate that the potency of preventive medicine, as hitherto applied, stood forth conspicuously declared, and that the promise of its future sovereignty might be discerned. Fifty years ago the deathrate of England and Wales stood at 22.5 per 1000 persons; in 1903 it had dropped to 15 4-a fall of 7.1 per 1000, representing, on the estimated population of 1903, a saving

Dr. Francis Galton, F.R.S., in a paper on physical records, suggested that on February 29 in each leap year there should be school reunions at which there might be an opportunity for reviving early friendships, and at which, at the same time, the anthropometric and other records of the pupils might be added to.

Each old boy would be represented by an envelope stored in the school library. This would contain his anthropometric record to date, and he would be given printed forms, containing a few well considered questions-health, profession, preferments, marriage, children, and general

remarks and would be asked to forward the filled-in forms to the school.

Many papers were read on infantile mortality and on municipal milk depôts.

In the unavoidable absence of Sir William Broadbent, a discussion on sanatoria for consumptives was opened by Dr. T. N. Kelynack, physician to the Mount Vernon Hospital for Consumption. To illustrate the enormous economic waste to the community caused by pulmonary tuberculosis, Dr. Kelynack mentioned that in the metropolitan district alone 40,000 people died of the disease every year, and the monetary loss to London had been estimated at 4,000,000l.

The provision of adequate assistance for the consumptive poor demanded urgent attention. Sanatoria or hygienic hospitals undoubtedly secured the best conditions for the arrest and alleviation of the disease. At present we were just muddling along, with no satisfactory organisation of our resources and no rational cooperation.

A resolution was unanimously adopted urging the Government to appoint a commission to deal with the subject of the sanatorium treatment of the poor.

The presidential address in the section of chemistry and bacteriology was given by Prof. R. T. Hewlett, It was a plea for the recognition of the place of the specialist in the various departments of public health. Proper administration required a medical officer and his sanitary staff, a bacteriologist, a chemist, and an engineer, all working cordially together to common end. For the smaller districts such a staff could be secured by grouping. Could they expect effective action if the medical officer was a


local practitioner who derived his livelihood by the goodwill of the local landlord?

Prof. Hewlett also denounced the way in which chemists were taking upon themselves the bacteriological examination of pathological material, and emphatically asserted that disease problems should be dealt with only by medical men. He also advocated that a course of biology should be obligatory for candidates for the associateship of the Institute of Chemistry taking the subject of biological chemistry.

An interesting discussion, opened by Dr. Newman, of Finsbury, was on the possibility of establishing a bacteriological standard of purity of milk. Dr. Newman suggested the following standards :-(a) not more than 24-25 degrees of total acidity at the time of sale, 1 degree being equivalent to 1 c.c. of deci-normal NaOH solution; (b) not an excess of pus or blood; (c) no B. coli, B. enteritidis, or B. enteritidis sporogenes; (d) non-virulent to guinea-pigs. All the speakers, including Dr. Allan Macfadyen, Prof. Kenwood, Dr. Savage, Colonel Firth, Mr. Revis and others, agreed that there was little possibility at present of fixing a standard, and Dr. Newman's suggestions did not obtain general acceptance.

Another discussion, on the relative efficiency of chemical and bacteriological methods for the examination of sewage effluents, was opened by Mr. Dibden and by Dr. Savage. There was a general agreement that chemical methods gave a better indication of proper purification than bacteriological ones, though, of course, bacteriological methods alone were of service in detecting species of microorganisms.

Licut. Nesfield, I.M.S., gave an interesting demonstration of a method devised by him for the sterilisation of drinking water during a campaign. He had found that chlorine in the proportion of 2 grams per 100 gallons acting for five minutes effectually destroyed the organisms of cholera, typhoid, and dysentery. His method consisted in carrying iron bottles of liquid chlorine, from which, by means of a valve, the requisite amount was liberated into the water. After five minutes a powder of sodium sulphite (2-2 grams) was added to the water, from which a double decomposition ensued, and the water was rendered absolutely tasteless. For the soldier on the march another method was devised, so that he could sterilise for himself a gallon of water. This consisted in adding to the vessel of water a tablet containing iodide and iodate of sodium. This resulted in the liberation of free iodine in the water, which acted in five minutes as an efficient germicide, and was then "killed" so that the water was rendered potable, by the addition of another tablet of sodium sulphite. In both processes the quantities of reagents employed are so small as to have no effect on the human economy; the methods are rapid, and the reagents, &c., portable.


ONE of the most peculiar appearances attending a total eclipse of the sun is that generally known as the "shadow bands." They are long dark bands, separated by white spaces, which are seen on the ground or sides of buildings just before and just after the total phase of an eclipse, moving rapidly. It is probable that they are not real bands, but are composed of dark patches which seem to the eye to make long bands. Their cause is not yet clearly known, as the observations in former eclipses are quite discordant. The undersigned is very desirous of obtaining observations of them at various stations along the line of totality, especially at places near the edge of the shadow, in order to compare with similar observations made by himself and others. The observations require no special instruments, and can be made by any careful person. Information is desired upon the following points: (1) the direction in which the bands lie; (2) the direction in which they move; (3) the velocity with which they move; (4) the width of the bands; and (5) their distance apart. All of these are likely to be different before and after the total phase, so that two sets of

observations are needed. The following suggestions are compiled from various sources.

Spread a white cloth or piece of canvas upon the ground in any convenient open space. It is well to spread two cloths or pieces of canvas, one to be used before, the other after, the total phase. Let each observer be furnished with several sticks, 4 feet to 6 feet long.

About three minutes before the time of totality, let the observer stand near the cloth with his back to the sun and watch the cloth intently. If bands or dark patches are seen, place one stick down in the direction in which they lie; after this is done place a second stick in the direction in which they are moving. Both of these operations should be done deliberately, not hurriedly, and the sticks left in position.

During the total phase the observer is free to enjoy the scene or make other observations, but it may be well to note if any bands can be seen during totality, as some have asserted.

At the close of totality the observer should be at the second cloth, or at another part of the single cloth (if he uses but one), and should repeat the observations made before totality, placing one stick down in the direction in which the bands lie, and another in the direction in which they move.

It will be seen that four sticks are needed for these observations. If two persons make the records, one should confine his attention to the direction in which the bands lie, the other to the direction in which they move. The bands are likely to be somewhat faint and poorly defined, so that extreme accuracy may not be possible.

The sticks should not be disturbed until after the eclipse, when their direction should be determined with as much care as possible, either by a compass or, still better, by a surveyor's theodolite if one is available. If neither compass nor theodolite is at hand, an estimate of the directions should be made.

The velocity with which the bands travel is more difficult to determine. The estimates vary from the speed of a man running to that of an express train. Several methods may be suggested :

(1) Let two persons work together, one having a watch with the seconds marked on the face. Let him mark time by calling out each second. The number of the second is not important, but a simple sound to mark the seconds is sufficient. Let the other observer watch the bands and see how many he can count per second.

(2) With one observer marking time as before, let the second observer note how many seconds elapse while a band is passing between two objects the distance apart of which is known.

(3) Let a person run a short distance with the bands and see if he can keep up with them. If not, let him estimate how much faster the bands are moving than he

can run.

(4) A mere guess at the speed is of some value.

The width of the bands and their distance apart can best be determined upon the cloths mentioned above, and it will add to the accuracy of the estimates if the cloths are divided by seams or in some other way into strips of known width. A carpenter's rule will aid the observer in making the estimates. The bands will probably be several inches wide and separated by spaces the width of which is the same or greater.

If the observer notices any other point connected with the bands, such as their colour, whether they are straight or wavy, whether they are continuous bands or made up of dark patches, whether they flicker or not, the information will be valuable. Still more valuable would be photographs of the bands as they pass over the ground or the side of a building or wall.

It is earnestly requested that anyone who will kindly attempt the above, either in whole or in part, will send his records to the undersigned. If for any reason the observation seems unsatisfactory, either because the bands were not as distinct as expected, or for any other reason, or if the record is only fragmentary, it will still be of value. The report should consist of a statement of the methods employed by the observer or observers in making the observations, and the results obtained, with any

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MY inclination has led me, in spite of a lively dread of incurring a charge of presumption, to address you principally on that profound and most subtle question, the nature and mode of formation of the photographic image. I am impelled to do so, not only because the subject is full of fascination and hopefulness, but because the wide topics of photographic methods or photographic applications would be quite unfittingly handled by the president you have chosen.

I would first direct your attention to Sir James Dewar's remarkable result that the photographic plate retains considerable power of forming the latent image at temperatures approaching the absolute zero-a result which, as I submit, compels us to regard the fundamental effects progressing in the film under the stimulus of light undulations as other than those of a purely chemical nature. But few, if any, instances of chemical combination or decomposition are known at so low a temperature. Purely chemical actions cease, indeed, at far higher temperatures, fluorine being among the few bodies which still show chemical activity at the comparatively elevated temperature of 180° C. In short, this result of Sir James Dewar's suggests that we must seek for the foundations of photographic action in some physical or intra-atomic effect which, as in the case of radio-activity or fluorescence, is not restricted to intervals of temperature over which active molecular vis viva prevails. It compels us to regard with doubt the rôle of oxidation or other chemical action as essential, but rather points to the view that such effects must be secondary or subsidiary. We feel, in a word, that we must turn for guidance to some purely photo-physical effect.

Here, in the first place, we naturally recall the views of Mr. Bose. This physicist would refer the formation of the image to a strain of the bromide of silver molecule under the electric force in the light wave, converting it into what might be regarded as an allotropic modification of the normal bromide which subsequently responds specially to the attack of the developer. The function of the sensitiser, according to this view, is to retard the recovery from strain. Bose obtained many suggestive parallels between the strain phenomena he was able to observe in silver and other substances under electromagnetic radiation and the behaviour of the photographic plate when subjected to long-continued exposure to light.

This theory, whatever it may have to recommend it, can hardly be regarded as offering a fundamental explanation. In the first place, we are left in the dark as to what the strain may be. It may mean many and various things. We know nothing as to the inner mechanism of its effects 1 Address to the Photographic Convention of the United Kingdom, 1905. By J. Joly, F.R.S.

upon subsequent chemical actions or at least we cannot correlate it with what is known of the physics of chemical activity. Finally, as will be seen later, it is hardly adequate to account for the varying degrees of stability which may apparently characterise the latent image. Still, there is much in Mr. Bose's work deserving of careful consideration. He has by no means exhausted the line of investigation he has originated.

Another theory has doubtless been in the minds of many. I have said we must seek guidance in some photo-physical phenomenon. There is one such which preeminently connects light and chemical phenomena through the intermediary of the effects of the former upon a component part of the atom. I refer to the phenomena of photo-electricity. It was ascertained by Hertz and his immediate successors that light has a remarkable power of discharging negative electrification from the surface of bodies-especially from certain substances. For long no explanation of the cause of this appeared. But the electron-the ubiquitous electron -is now known with considerable certainty to be responsible. The effect of the electric force in the light wave is to direct or assist the electrons contained in the substance to escape from the surface of the body. Each electron carries away a very small charge of negative electrification. If, then, a body is originally charged negatively, it will be gradually discharged by this convective process. If it is not charged to start with, the electrons will still be liberated at the surface of the body, and this will acquire a positive charge. If the body is positively charged at first, we cannot discharge it by illumination.

It would be superfluous for me to speak here of the nature of electrons or of the various modes in which their presence may be detected. Suffice it to say, in further connection with the Hertz effect, that when projected among gaseous molecules the electron soon attaches itself to one of these. In other words, it ionises a molecule of the gas or confers its electric charge upon it. The gaseous molecule may even be itself disrupted by impact of the electron if this is moving fast enough and left bereft of an electron.

We must note that such ionisation may be regarded as conferring potential chemical properties upon the molecules of the gas and upon the substance whence the electrons are derived. Similar ionisation under electric forces enters, as we now believe, into all the chemical effects progressing in the galvanic cell, and, indeed, generally in ionised solutants.

An experiment will best illustrate the principles I wish to remind you of. A clean aluminium plate, carefully insulated by a sulphur support, is faced by a sheet of copper-wire-gauze placed a couple of centimetres away from it. The gauze is maintained at a high positive potential by this dry pile. A sensitive gold-leaf electroscope is attached to the aluminium plate, and its image thrown upon the screen. I now turn the light from this arc lamp upon the wire gauze, through which it in part passes and shines upon the aluminium plate. The electroscope at once charges up rapidly. There is a liberation of negative electrons at the surface of the aluminium; these, under the attraction of the positive body, are rapidly removed as ions, and the electroscope charges up positively.

Again, if I simply electrify negatively this aluminium plate so that the leaves of the attached electroscope diverge widely, and now expose it to the rays from the arc lamp, the charge, as you see, is very rapidly dissipated. With positive electrification of the aluminium there is no effect

attendant on the illumination.

Thus from the work of Hertz and his successors we know that light, and more generally what we call actinic light, is an effective means of freeing the electron from certain substances. In short, our photographic agent, light, has the power of evoking from certain substances the electron which is so potent a factor in most, if not in all, chemical effects. I have not time here to refer to the work of Elster and Geitel whereby they have shown that this action is to be traced to the electric force in the light wave, but must turn to the probable bearing of this phenomenon on the familiar facts of photography. I assume that the experiment I have shown you is the most fundamental photographic experiment which it is now in our power to make.

We must first ask from what substances can light liberate the electron. There are many-metals as well as non-metals and liquids. It is a very general phenomenon, and must operate widely throughout nature. But what chiefly concerns the present consideration is the fact that the haloid salts of silver are vigorously photo-electric, and, it is suggested, possess, according to Schmidt, an activity in the descending order bromide, chloride, iodide. This is, in other words, their order of activity as ionisers (under the proper conditions) when exposed to ultra-violet light. Photographers will recognise that this is also the order of their photographic sensitiveness.

Another class of bodies also concerns our subject :-the special sensitisers used by the photographer to modify the spectral distribution of sensibility of the haloid salts, e.g. eosine, fuchsine, cyanine. These again are electron-producers under light stimulus. Now it has been shown by Stoletow, Hallwachs, and Elster and Geitel that there is an intimate connection between photo-electric activity and the absorption of light by the substance, and, indeed, that the particular wave-lengths absorbed by the substance are those which are effective in liberating the electrons. Thus we have strong reason for believing that the vigorous photo-electric activity displayed by the special sensitisers must be dependent upon their colour absorption. You will recognise that this is just the connection between their photographic effects and their behaviour towards light.

There is yet another suggestive parallel. I referred to the observation of Sir James Dewar as to the continued sensitiveness of the photographic film at the lowest attained extremes of temperature, and drew the inference that the fundamental photographic action must be of intra-atomic nature, and not dependent upon the vis viva of the molecule or atom. In then seeking the origin of photographic action in photo-electric phenomena we naturally ask, Are these latter phenomena also traceable down to low temperatures? If they are, we are entitled to look upon this fact as a qualifying characteristic or as another link in the chain of evidence connecting photographic with photoelectric activity.

I have quite recently, with the aid of liquid air kindly supplied to me by Mr. Moss, and made in the laboratory of this society, tested the photo-sensibility of aluminium and also of silver bromide down to temperatures approaching that of the liquid air. The mode of observation is essentially that of Schmidt-what he terms his static method. The substance undergoing observation is, however, contained at the bottom of a thin copper tube, 5 cm. in diameter, which is immersed to a depth of about 10 cm. in liquid air. The tube is closed above by a paraffin stopper which carries a thin quartz window as well as the sulphur tubes through which the connections pass. The air within is very carefully dried by phosphorus pentoxide before the experiment. The arc light was used as source of illumination. It was found that a vigorous photo-electric effect continued in the case of the clean aluminium. In the case of the silver bromide a distinct photo-electric effect was still observed. I have not had leisure to make, as yet, any trustworthy estimate of the percentage effect at this temperature in the case of either substance. Nor have I determined the temperature accurately. The latter may be taken as roughly about - 150° C. Sir James Dewar's actual measurements afforded twenty per cent, of the normal photographic effect at 180° C. and ten per cent, at the temperature of -252°5 C.

With this much to go upon, and the important additional fact that the electronic discharge-as from the X-ray tube or from radium--generates the latent image, I think we are fully entitled to suggest as a legitimate lead to experiment the hypothesis that the beginnings of photographic action involve an electronic discharge from the light sensitive molecule; in other words that the latent image is built up of ionised atoms or molecules the result of the photo-electric effect on the illuminated silver haloid, upon which ionised atoms the chemical effects of the developer are subsequently directed. It may be that the liberated electrons ionise molecules not directly affected, or it may be that in their liberation they disrupt complex molecules built up in the ripening of the emulsion. With the amount we have to go upon we cannot venture to particularise. It

will be said that such an action must be in part of the nature of a chemical effect. This must be admitted, and, in so far as the re-arrangement of molecular fabrics is involved, the result will doubtless be controlled by temperature conditions. The facts observed by Sir James Dewar support this. But there is involved a fundamental processthe liberation of the electron by the electric force in the light wave, which is a physical effect, and which, upon the hypothesis of its reality as a factor in forming the latent image, appears to explain completely the outstanding photographic sensitiveness of the film at temperatures far below those at which chemical actions in general cease.

Again, we may assume that the electron-producing power of the special sensitiser or dye for the particular ray it absorbs is responsible, or responsible in part, for the special sensitiveness it confers upon the film. Sir Wm. Abney has shown that these sensitisers are active even if laid on as a varnish on the sensitive surface and removed before development. It must be remembered, however, that at temperatures of about 50° these sensitisers lose much of their influence on the film. [See a paper by me read before the convention in 1894.]

It appears to me that on these views the curious phenomenon of recurrent reversals does not present a problem hopeless of explanation. The process of photoionisation constituting the latent image, where the ion is probably not immediately neutralised by chemical combination, presents features akin to the charging of a capacity-say a leyden jar. There may be a rising potential between the groups of ions until ultimately a point is attained when there is a spontaneous neutralisation. I may observe that the phenomena of reversal appear to indicate that the change upon the silver bromide molecule, whatever be its nature, is one of gradually increasing intensity, and finally attains a maximum when a return to the original condition occurs. The maximum is the point of most intense developable image. It is probable that the sensitiser-in this case the gelatin in which the bromide of silver is immersed—plays a part in the conditions of stability which are involved.

Of great interest in all our considerations and theories is the recent work of Prof. Wood on photographic reversal. The result of this work is as I take it-to show that the stability of the latent image may be very various according to the mode of its formation. Thus it appears that the sort of latent effect which is produced by pressure or friction is the least stable of any. This may be reversed or wiped out by the application of any other known form of photographic stimulus. Thus an exposure to X-rays will obliterate it, or a very brief exposure to light. The latent image arising from X-rays is next in order of increasing stability. Light action will remove this. Third in order is a very brief light-shock or sudden flash. This cannot be reversed by any of the foregoing modes of stimulation, but a long-continued undulatory stimulus, as from lamp-light, will reverse it. Last and most stable of all is the gradually built-up configuration due to long-continued light exposure. This can only be reversed by overdoing it according to the known facts of recurrent reversal. Prof. Wood takes occasion to remark that these phenomena are in bad agreement with the strain theory of Mr. Bose. We have, in fact, but the one resource-the allotropic modification of the haloid-whereby to explain all these orders of stability. It appears to me that the elasticity of the electronic theory is greater. The state of the ionised system may be very various according as it arises from continued rhythmic effects or from unorganised shocks. The ionisation due to X-rays or to friction will probably be quite unorganised, that due to light more or less stable according to the gradual and gentle nature of the forces at work. I think we are entitled to conclude that on the whole there is nothing in Prof. Wood's beautiful experiments opposed to the photo-electric origin of photographic effects, but that they rather fall in with what might be anticipated.

When we look for further support to the views I have laid before you we are confronted with many difficulties. I have not as yet detected any electronic discharge from the film under light stimulus. This may be due to my defective experiments, or to a fact noted by Elster and Geitel concerning the photo-electric properties of gelatin.

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