Slike strani

own methods; and as none will ever become photomicrographers who have not some ingenuity and enthusiasm, it is only needful that they be set to work, and they will undoubtedly find their own "best methods."

This treatise is too general to expect from it more than useful and suggestive hints on the subject of the preparation and mounting of objects; and the same may be said as to the history of the microscope, which is nevertheless given in an interesting and useful manner. The book will undoubtedly attract many readers, and it will afford help to many who are seeking it; but we respectfully doubt whether it will enable the elementary reader to fully follow the diffraction theory of microscopic vision, so as to be able to understand its application to the wide range of subjects supposed to be dealt with from that point of view by this sumptuous treatise.

W. H. DALLinger.


The Earth's History: an Introduction to Modern Geology. By R. D. Roberts, M.A. (Camb.), D.Sc. (Lond.). With Coloured Maps and Illustrations. (London: John Murray, 1893.)

THIS is not a large book, and a slightly less ambitious

title might have been more appropriate. Certainly it is an introduction to the study of modern geology rather than a history of the earth, for the latter is regarded from a limited point of view. But from the page preceding the frontispiece it appears that the volume is one of a series of "University Extension Manuals." It partakes, therefore, of the advantages and disadvantages of this method of disseminating knowledge.

The topics treated by Dr. Roberts are the progress of geological thought: the beginnings of the earth's history:

the modifications of its surface due to forces destructive and reproductive: the movements of its crust, including the action of volcanoes. Finally he deals with the formation of rock masses, and attempts to give though of necessity this subject is very imperfectly treated-some idea of the evolution of the British Islands.

The materials employed by the author are not generally novel, for one text-book must draw from much the same storehouse as another, but Dr. Roberts has a lucid and pleasant method of statement, gained no doubt by his experience in the lecture room. One point, however, though it relates to a well-worn subject, will be fresh to most readers. In speaking of the submergence of the so-called Temple of Serapis at Puzzuoli, Dr. Roberts cites a passage from the Acts of Peter and Paul, an apocryphal booklet, to which attention was drawn a few months since by Mr. Thomson (Geol. Mag., 1892, p. 282). This states that Pontioli (Puteoli, now Puzzuoli) was submerged as a punishment for the martyrdom of Dioscurus. "They all see that city Pontioli sunk into the sea-shore about one fathom, and there it is unto this day for a remembrance under the sea." On which passage Dr. Roberts observes that when the Acts was written, “Puzzuoli was under water, and had been so for so long a time that the memory of the actual events had been lost and replaced by the tradition recorded in the Acts." At first sight this, as he says, seems in favour of the submergence having occurred

"between the third and fifth centuries, and pr earlier than the fourth."

This passage certainly makes it probable that the mergence began at a rather early time, but it is no matter to fix the date of any passage in these Parts of the book are believed to be as old as the se century, while others are not earlier than the fifth cen The book, also, was not of Western but of Ease origin. Had the book been written in Italy then, withstanding its other absurdities, some weight mig attached to a topographical reference; but these, as compiled at a distance, and by obviously ignorant pe seriously impair its credit. It is also needful to that this story forms part of the later recensions and is merely founded on some vague tradition of change of e in the neighbourhood. In any case, Dr. Roberts seit to go a little too far in saying "this would allow abr ten centuries, during which the marble columns under water exposed to the action of the living molass Hardly so; this tradition at most would not take us beja the first submergence, that indicated by the bra water deposit at the base of the pillars. Over thi an irregular mass of volcanic ash, which was cover a calcareous tufa, in places full four feet thick. T former, of course, may have accumulated in a few br

but the latter must indicate a considerable time. temple, also, must have been in complete ruin beforeshowers of ashes fell-which also would require time that Dr. Roberts perhaps would have done better to: adhered to the more cautious statement in Mr. Thos letter, and not claimed quite so long a period it maximum submergence.

Within the limits, which the necessities of the impose, the book is well conceived and well exe though we cannot help doubting the wisdom of encoun ing,by manuals necessarily partial and incomplete str to imagine that they have really mastered a subject any rate, it should be frankly admitted that this, b useful and interesting, is not education.


The Health Officer's Pocket-Book. By E. F. W M.D., D.P.H. (London: Crosby Lockwood Son, 1893.)

THIS is a work the object of which is to provid able and well-bound book of reference, to w health officer may turn at any moment for mos facts, formulæ, and data required in his daily pr and while one cannot give unqualified assent to 21 loughby's contention that such a book is indispe one is prepared to acknowledge that it may prot useful. It is not easy, however, to conceive the Counder which a health officer is called upon to take or to give advice, at a moment's notice, upor ? remote from the routine practice of his duties tha ever find it necessary to carry about, for consu pocket-book of abstruse sanitary facts and fort legal enactments. If such a work is inds the author would have done well to restrict 5 ness somewhat, and more especially since 20 have achieved this by the omission of a great matter which is, on the face of it, foreign to the p of the book. To instance such :-The parts w genous and non-nitrogenous food stuffs play in the economy; the origin and nature of cyclones; ag

f discursive material upon vital statistics; and a host of ementary hygienic facts with which every sanitarian is onversant, are none of them points it can ever be necestry for the health officer to carry about with him for hasty ference.

The most useful sections, and those which most justify e motif of the book, are the following:-Those which eal with mathematical problems, and set forth useful gebraical and trigonometrical formulæ, together with a w logarithm tables; that upon demography and vital atistics; and the serviceable abstract of sanitary law, which corresponding or similar sections of the Public ealth Act, 1875, and the Public Health (London) Act, 91, are considered side by side.

There is very little in the book which is not correct and to date, save that which refers to the subject of water alysis. This contains many errors, and, since the ility of its introduction is very questionable, it is regretle that it mars the all-round accuracy of the work. In is section Dr. Willoughby gives several results of his n analyses, and those who are familiar with the bject will find their experiences much at variance with

⚫ writer's.

In what he calls a typical sample of rain-water he ind 063 grains per gallon of nitrates as HNO,, and 14 and o'172 parts per million of "ammonia and lbuminoid ammonia" respectively; in river-water at tchford he found no nitrates, not even a fraction of a rt per million, and the "ammonia" and "albuminoidmonia" were o 08 and o'16 (parts per million) respectly. Loch Katrine water is, moreover, credited (and anklyn is quoted as the authority) with o'008 parts million of" albuminoid-ammonia," and with 0'004 of mmonia ;" and the former is said to correspond to 056 grains per gallon !

While unquestionably the work contains some material ich will make it useful to the health-officer, the health dent will find much in it which he may peruse with vantage.

gler's Botanische Jahrbücher für Systematik, Pflanzeneschichte und Pflanzengeographie. (Leipzig: W. Engelmann.)

NCE Dr. A. Engler's appointment to the post of ector of the Berlin Botanical Garden and Museum, 5 periodical has become the organ of the very active tí of botanists of that establishment; and the comatively recent German colonial policy has revived interest in systematic and economic botany, to ich it is devoted chiefly. Vols. xv. and xvi. being published concurrently. This publica1 is partly devoted to original work and partly to a ew of contemporary botanical literature. The fifteenth ume is largely taken up by contributions to the a of tropical Africa, in the form of an elaboration by ious botanists of the extensive collections made by merous German travellers. Quite a host of new species described, but, truth to say, nothing very remarkable new generic types. Hybophrynium is a new genus of tamineæ, near Trachyphrynium, with which it was erically associated by Bentham and Hooker; and the ideæ, elaborated by Engler himself, include two or ee new genera. Pseudohydrosme is characterised by a e, almost truncate spathe and a spadix without any ninal naked continuation.

Dr. J. Urban, who has been for some years engaged collecting materials for a general flora of the West ies, contributes Additamenta ad Cognitionem Floræ ia Occidentalis," a critical work, both from a anical and a literary standpoint. No new genera are ribed.

ne of the most interesting articles in the sixteenth me is by Dr. O. Warburg, on the mountain plants of ser Wilhelm's-Land, New Guinea. The collection of

plants dealt with consisted of only fifty-three species, whereof thirty-two were supposed to be endemic, though the material of a few was insufficient for description. Two new genera are described, namely, Hellwigia pulchra, a pretty scitamineous plant, and Zoelleria, a singular boragineous plant, described as having ten nutlets in the place of the usual four! Among the new species are five rhododendrons, and the most noteworthy feature of the collection was the absence of essentially Australian types.

Another paper of general interest is Dr. Kränzlin's "Beiträge zu einer Monographie der Gattung Habenaria," excluding Platanthera, united with Habenaria by some botanists, 347 species are described; and they are spread over nearly the whole area inhabited by orchids.

Dr. Carl Bolle's "Botanische Rückblicke auf die Inseln Lanzarote und Fuertaventura" is a pleasantly written essay on the indigenous and cultivated plants of these islands. The "Jahrbücher" contain many other valuable articles. W. B. H.

Descriptive Geometry Models for the use of Students in Schools and Colleges. Designed by T. Jones, M.I.M.E. (Moss Side, Manchester.)

THE models are six in number. They are intended to show a line (1) by its projections, (2) by its traces; the inclination of an oblique plane (3) to the vertical plane, (4) to the horizontal plane; and to determine the angle (5) between two intersecting lines (6) between two planes. They are accompanied by hints for fixing and studying the models, and with a useful list of problems suggested as exercises for students. The clearness of the explanations, the simplicity of the constructive apparatus, and the compactness of the arrangements (all being contained in a handy cardboard box) commend Mr. Jones's work to students of solid geometry.


[The Editor does not hold himself responsible for opinions expressed by his correspondents. Neither can he undertake to return, or to correspond with the writers of, rejected manuscripts intended for this or any other part of NATURE. No notice is taken of anonymous communications.]

Lion-Tiger Hybrids.

I HAVE read Dr. Ball's account of this subject in the issue of NATURE for February 23, 1893, and beg leave to call attention to the fact that the University of Cambridge possesses the skeleton and the stuffed skin of an adult hybrid between a lion and a tigress. I am able to supply the following information (which I have verified so far as it was possible) with regard to this specimen from a contemporary MS., entitled "Notice of Melson, then an Undergraduate at Trinity College. This MS. the Lion-tiger which died in Cambridge, March 1833,” by J. B. no doubt contains the substance of a paper by Mr. Melson, which was communicated by Dr. Haviland to the Cambridge Philosophical Society, May 6, 1833. The paper was unfortu nately not printed in the Transactions of the Society.

The Cambridge specimen, like those mentioned by Dr. Ball, was procured from the menagerie of Mr. Atkins. It was about six years old, and for some time previous to its death had been affected with paralysis of its hind quarters, arising probably from a distortion of the lower thoracic region of the vertebral column [which is still a marked feature of the actual skeleton]. Although inferior in size to either of its parents, the animal appeared to have attained its full dimensions. The shape of the head resembled that of the father (the lion), whilst the form of the body was more similar to that of the tigress. The body was faintly striped, while the prevailing shade was "of a dingy lion colour." The animal had neither a mane nor a tuft at the end of its tail.

The specimen was a female, and Mr. Melson states that "all the individuals of this hybrid race have as yet been females." The orifice of the vagina was smaller than in the tigress; and

the uterus was "merely rudimentary and nothing more than a membranous tube terminating in the two fallopian tubes." The ovaries were normal in appearance, though very much smaller than those of the full-grown lioness or tigress.

The extreme length of the skull, from the end of the occipital crest to the end of the præmaxillæ, of the specimen now in the Cambridge Museum, is 290 mm. ; the distance between the foramen magnum and the end of the præmaxillæ is 235 mm. ; and the extreme zygomatic breadth is 190 mm. The ascending process of the maxilla ends at a point 3 mm. in front of the posterior end of the nasal bones, and has a somewhat rounded termination. In these characters the skull of the hybrid resembles that of the lion much more closely than that of the tiger. S. F. HARMER.

University Museum of Zoology, Cambridge,
February 27.

Travelling of Roots.

THE mode in which roots travel in pursuit of food (moisture) is often remarkable. Innumerable instances have been published. But I think the inclosed is one of the most striking which I have come across. The specimen kindly sent to the Kew Museum by the vicar of Petersham is most extraordinary. The roots seem to have behaved more like the mycelium of a fungus than an ordinary axial structure. W. T. THISELTON-DYER.

Royal Gardens, Kew, Feb. 24. Memorandum by the Rev. W. H. Oxley, Vicar of Petersham, dated February 16, 1893.

Roots of a Wistaria from the dining-room of Eden House, Ham, just demolished.

The root entered the room by a very small chink in the side of the window, near the ceiling, and on removing the paper, which had not been disturbed for many years, from the walls (of the room about 14ft. square) the whole of the plaster beneath the paper was found covered with a fine network of roots spreading all round the room. The specimen is about one-third of the whole roots and the stem where it entered the room. There was not the faintest appearance of anything of the sort on the surface of the wall paper to give rise to the suspicion of these roots being there, and the room was continually inhabited, with fires, &c.

The Flight of Birds.

WITH reference to an extract from Science on the flight of birds, which appeared in your "Notes" of February 16, I agree with the writer of that extract that the rapidity with which the generality of birds travel is often considerably over-estimated.

Some few months ago, whilst crossing, by G. W. R. express, the moors of Bridgewater Level in Somerset, a couple of turtle-doves rose at a distance of about eighty yards from the train, and flew for a considerable distance in a line nearly parallel with the rails.

I observed them with much interest, for I wished to have some comparison of their power of flight with that of some "homing" pigeons in my possession, and perceived that they were being slowly overtaken. They must have flown fairly parallel with the line of rails for at least 500 yards, and finally bore away northward. We must have been travelling at about forty miles an hour at the time, so that their speed would have been a little less than that. I was the more surprised at this as I had had "homing" pigeons, trained by myself, which, on a clear, calm day, had flown from the Quantock Hills to Taunton (a distance of seven miles) in less than eight minutes-a quite superior rate of flight, which, however, I do not think they would continue for a long distance. The Columbacei generally may be considered good flyers; the turtle, however, I believe from observation to be somewhat below the average standard of excellence. It certainly cannot be compared with the Passenger Pigeon of America, which has frequently been killed in the neighbourhood of New York with Carolina rice still undigested in its crop—having probably accomplished a journey of between 300 and 400 miles in about six hours, giving the high record of sixty miles an hour for six hours in succession. My own impression is that there is a great difference in the speeds of various orders and tribes of birds. I have repeatedly observed the fieldfare, which is a fairly strong flyer, overtaken by trains of which I have been an occupant, and which could not have been

travelling more than forty miles an hour. On the other han I have witnessed the pursuit of a wood-pigeon or cash a hawk, in which both birds exhibited powers of flight w might seem incredible to persons unobservant of nature. I instance I should have estimated the speed of the pigeon, w was straining every muscle to reach the shelter of a bette ber, to be about sixty miles an hour; whilst that of the h which flew with little effort, could not I think have been than eighty, during the brief period that they were within sight. I should be glad to hear from any of your corresponde their opinion as to the rapidity of flight in the Ra (British). HERBERT WITHING A

Taunton, February 22.

The Niagara Spray Clouds.

I Do not remember having seen anywhere a reference to fact that the spray clouds of Niagara exhibit an ice bow in cr frosty weather.

I had an opportunity last week of seeing a very fine comp bow, the inner one, the outer being absent.

There was no trace of the mock suns or of the bands of w light usually present; though I have seen ice bows withert latter, I have never seen one before without any trace ef suns; these are generally accounted for by supposing the ence of hexagonal ice prisms. I would suggest the inf that the ice crystals in the Niagara spray clouds are not pr but rhombs. CHAS. A. CARUS-WILS McGill University, Montreal, February 6.

British New Guinea.

IN NATURE (vol. xlvii. p. 345) Mr. H. 0. For has a lenient review of Mr. J. P. Thomson's "British Guinea," in which he reproduces a figure of four natives the original they are called "native mountaineers" (p. 95 a matter of fact only the two central men are mountaineers two outermost being coast natives who acted as decoys to the timid highlanders to submit to being photographed. Thomson has a reprehensible habit of inserting figures wh while they illustrate the contiguous text, really belong different part of British New Guinea than that there with. I fancy Mr. Forbes has been deceived in this respe the last figure which appears in the review is entitled by Thomson Native Ornaments" (p. 120), and, though oc in his description of the Fly River district, represent, it not mistaken, Papuan Gulf natives, most probably Motuans. ALFRED C. HADE

I QUITE agree with Prof. Haddon's remarks above, whine have been good enough to submit to me, with regard mountaineers of the interior of New Guinea. They enter details which, in an already over-long review, I had to for. There is no doubt about the right-hand figure 13. being not a mountaineer. I was less confident about the on the left hand. The two central figures recall to me per the people of Uburukara, of whom I took photographs in the plates of which were ruined during my disastrous march the Goldie, and it was they who specially attracted my a With regard to the "Fly River" natives, I have never fortune to see any of them, but I certainly took the cent to be one, while remarking to myself the likeness of the hand man to a Motuan-to men with whom he could be ri in any village indeed between the Gulf and Kerepunu 104, Philbeach Gardens, S. W.


Some Lake Basins in France.

I REGRET that, through some inadvertence on my pat name of the author of the "Atlas des Lacs Français," mer in my letter (p. 341) is wrongly printed. It should becque. In a letter received from M. Delebecque, he me that "the direction of the arrow on the map of Lake is not exactly N., but N. 7° W." He informs me also curious funnel-shaped hole at the northern end of the Annecy, which I suggest may be a submerged swa is the site of a spring. This fact, however, need not ter my suggestion, because the changes in level might conver was once a swallow-hole into a spring. At present one time flows up from the dolinas of the Julian A another it drains off down them. T. G. Boys



WHEN I was honoured by the invitation to deliver this lecture I felt some doubt as to my ability to find a subject which should be suitable, for there is a prevailing idea that in addressing the operative classes, it is necessary to speak only of some practical subject which bears immediately upon the most important industry of the place in which the lecture is being delivered; but it seems to me that this is a polite suggestion that the audience are unable to be interested by any subject except that particular one which occupies them daily. Now though I am a comparative stranger in Scotland I have heard quite enough, and I know quite enough, of the superiority of the education of you, who have the good fortune to live in this the most beautiful half of Great Britain, to be aware that, as is the case with all highly-educated men, you are able to take a keen and genuine interest in many subjects, and that I had better choose one to which I have specially devoted myself, if I do not wish to expose myself to the risk of being corrected. I will ask you therefore in imagination to leave your daily occupation and come with me into the physical laboratory, where, by the exercise of the art of the experimentalist, problems which might seem to be impossible are continually being solved. I wish as an experimentalist to present to you an example of experimental enquiry.

Let us suppose that for some reason we wish to examine

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carefully and accurately some moving object travelling, if you will, at so great a speed that, observed in the ordinary way, it appears as a mere blur, or perhaps at a speed so tremendous that it cannot be seen at all. In such a case, in order to get a clear view of the moving body we may either look through an aperture which is only opened for a moment as the body passes by, or we may suddenly illuminate the object by a flash of light when it is in a position in which it may be seen. If in either of these cases the hole is open, or the illumination lasts so short a time that the object has no time to move appreciably while it is in this way brought into view, we get what may in ordinary language be called an instantaneous impression and the object appears clear, sharp, and at rest. In the same way if we wish, with the object of obtaining a permanent record, to photograph a moving body we must either allow the eye of the camera to see through a hole for a moment, i.e. we must use a rapid shutter, and many such are well known, or we must, keeping the photographic plate exposed and the object in the dark, make a flash of light at the right time. As before, if the shutter is open or the flash lasts so short a time that the object cannot move appreciably injthe time, then, if any impression is left at all it will be sharp, clear, and the same as if the body were at rest. The first method, that of the shutter, I do not intend to speak about to-night, but as, owing to the kindness of Mr. F. J. Smith, I have with me the most beautiful example that I have seen of what can be done by this method, I thought perhaps I should do well to show it. Mr. Smith was in an express train near Taunton, travelling at forty miles an hour, and when another express was coming up in the opposite direction at sixty miles an hour, i.e. approaching him at one hundred miles an hour, he aimed his camera at it and let a shutter of his own construction open and shut so quickly that the approaching train was photographed sharply. There is a special interest about this photograph; it shows one of the now extinct broadgauge engines on the road. However, this is an example of the method which we shall not consider this evening. 1 Lecture delivered at the Edinburgh meeting of the British Association by

C. V. Boys, F.R.S.

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For our purpose we require what is called instantaneous illumination, a flash of light. It is of course obvious that it depends entirely upon the speed of the object and the sharpness required, whether any particular flash is instantaneous enough. No flash is absolutely instan

taneous, though some may last a very short time.

For instance, a flash of burning magnesium powder lasts so short a time that it may be used for the purpose of portraiture, and while it lasts even the eye itself has no time to change. second slide (Fig. A) is a photograph of the eye of Mr. The lower part of the

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Colebrook after he had been some minutes in a dark room, taken by the magnesium flash; the upper part is the same eye taken in daylight. The pupil is seen fully dilated and the eyelid has not had time to come down, and so we might reasonably say that the flash was instantaneous; it was for the purpose practically instantaneous. Yet when I make this large clock-face four feet across revolve at so moderate a speed that the periphery is only travelling at forty miles an hour and illuminate it by a magnesium flash you see no figures or marks at all, only a blur. Thus the magnesium flash, which for one purpose is practically instantaneous, is, tested in this simple way, found to last a long time. Let me now, following Lord Rayleigh, contrast the effect of the magnesium flash with that of a powerful electric spark. At each spark the clock-face appears brilliantly illuminated and absolutely at rest and clear, and if it were not that I could at once illuminate it by ordinary light it would be difficult to believe that it was still in motion.

The electric spark has been often used to produce a flash by means of which phenomena have been observed which we ordinarily cannot see. For instance, Mr. Worthington has in this way seen and drawn the exact form of the splash produced by a falling drop of liquid.

and others have used the illumination produced by an Mr. Chichester Bell, Lord Rayleigh, Mr. F. J. Smith, electric spark to photograph phenomena which they were investigating. I am able to show one of Lord Rayleigh's, a breaking soap-bubble, in which the retreating edge, travelling something like thirty miles an hour, is seen with all the accuracy and sharpness that is possible with a stationary for the purpose of physiological enquiry, taking a row of object. Mr. F. J. Smith has extended the use of sparks photographs on a moving plate at intervals that can be arranged to suit the subject, and is thus putting in the hands of the much-abused experimental physiologist a very powerful weapon of research. I had hoped to show one of these series of an untechnical character, to wit, a series taken of a cat held by its four legs in an inverted position and allowed to drop. The cat, as every one is aware, seems to do that which is known to be dynamically impossible, namely, on being dropped upside down to turn round after being let go and to come down the right way up. The process can be followed by means of one of Mr. Smith's multiple spark photographs. However, his cats do not seem to like the experiments, and he has in consequence had so much trouble with them that his results,

while they are of interest, are not, up to the present, suitable for exhibition.

Let me now return to single spark photographs. We have seen that the magnesium flash, which for the purpose of portraiture is practically instantaneous, yet fails to appear so when so moderate a speed as forty miles an hour (and indeed a far lower speed) is used for the purpose of examining it. Is anything of the kind true in the case of the electric spark? Will the spark, by which we saw the clock-face absolutely sharp, after all fail to give a sharp view when tested by a much higher speed? I have taken such a spark and attempted (though I knew what the result would be) to photograph by its light the bullet of a magazine rifle passing by at the rate of about 2100 feet a second, or, what is the same thing, at about 1400 miles an

FIG. 1.

hour; the result (Fig. 1) shows not a clear sharp bullet but a blur; the spark lasted so long a time that this bullet was actually able to travel half an inch or so while the illumination lasted. Thus we see, that if we wish to examine bullets, &c., in their flight, any electric spark will not necessarily do. We shall have to get a spark which while it gives enough light to act on the plate yet lasts so short a time that even a rifle bullet cannot move an appreciable distance during the time that it is in existence. A knowledge of electrical principles enables one to modify the electrical apparatus employed to make this spark in such a manner that its duration may be greatly reduced without, at the same time, a very great sacrifice of light; but while this may be done it is important to be able to observe how long the spark actually lasts, when made by apparatus altered little by little in the proper manner. The desired information is at once given by the revolving mirror. For instance, every one is aware how, by a turn of the wrist, one may reflect a beam of sunlight from a piece of looking-glass so as to travel up the street at a most tremendous velocity; but suppose that, instead of being moved by a mere turn of the wrist, the mirror is made to rotate on an axle by mechanical means at an enormous speed; then, just as the rotation is more rapid, so will the beam of light travel at a higher speed. In the particular case that I am going now to bring before your notice, a small mirror of hardened steel was made by Mr. Colebrook, the mechanical assistant in the physical laboratory at South Kensington, mounted so beautifully that it would run at the enormous speed of 1000 turns a second (not 1000 a minute) without giving any trouble. The light from the spark was focussed by the mirror upon a photographic plate. Now if the light were really instantaneous, the image would be as clear and sharp as if the mirror were at rest; if, on the other hand, it lasted long enough for the image to be carried an appreciable distance, then the photograph would show a band of light drawn out to this distance. The mirror is now placed on the front of the platform, and a beam of electric light is focussed by it upon the screen, from which it is distant about 20 feet. Now that I turn the mirror slowly, you see the spot of light drawn out into a band reaching across

the screen, and this is described over and over again the mirror revolves. Let us suppose that the mirror is t volving once a second, then it is easy to show that t spot of light is travelling at about 250 feet a second. not difficult therefore to see that if the mirror is revolving 1000 times as fast, the spot of light will traverse the scree 1000 times as fast also, i.e., about 250,000 feet a seco or 160,000 miles an hour-a speed which is 200 times as great as that of a Martini-Henry bullet, while such a be only travels 14 times as fast as an express train. Your see, then, that it is not difficult to observe how long a spark lasts when its image can be whirled along at such a speed as this. I have now started the electro-motor, and the mirror is turning more and more rapidly. Now it gives a musical note of the same pitch as that given by the tuning-fork I am bowing; it is therefore turning 1 times a second. It is now giving a higher note, i turning faster and faster, until at last it gives the octave. at which time it is turning 1028 turns a second. The band of light on the screen is produced by a spot as travelling at a still higher speed than that which I ha just mentioned. I had hoped to have shown with apparatus the actual experiment of drawing out the as parently instantaneous flash of an electric spark in band of light, but I found that while it was easy to show the experiment in a small room, the amount of light not sufficient to be seen in a great room like this. I as therefore be content to show one or two of the ph graphs which were taken lately in the physical laborato at South Kensington by two of the students, Mr. E and Mr. Stansfield, whom I now take the opportunity thanking. The next slide shows the drawn-out band p particular spark made between magnesium termina the discharge of a condenser of 23 square feet of win glass, the spark being inch long. Below the draw band I have drawn a scale of millionths of a sem If the spark had been instantaneous it would have peared as a fine vertical line. This line, howeve been drawn sideways to an extent depending on the tion of the spark. The spark, except at the ends, is ext in rather less than one-millionth of a second, but ends remain alight like two stars, being drawn o consequence into two lines, which have lasted a measured by the scale, as long as six or seven million of a second. Such a light is, therefore, seen to when tested with this very powerful instrument so long that it seems absurd to call it instantaneous. It lasts long for the purpose of bullet photography. In order get sparks of shorter duration it is necessary to abs the metal magnesium, in spite of the brilliant ph graphic effect of the two ends of the spark betwe knobs of this material, it is well to avoid all easily volati metals, such as brass, because of the zinc that it cont and instead to employ beads of copper or of platinum the second place, the duration of the spark proper, wh in the last case was nearly a millionth of a second be reduced by (1) reducing the size of the condenser, one must not go too far, as the light is reduced (2) by replacing any wire through which the dischay may have taken place by broad bands of copper as

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short as possible, this has the further advantage of increasing the light; and (3) the light may be increased without much change being made in the duration by making a second gap in the discharge circuit, the spark in which, however, must be hidden from the plate. shows the trail of the best spark for the purpose of

FIG. 2

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