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petroleum mining expert he visited Egypt, Russia, Galicia, and Rumania. As a journalist Prof. Louis had a marvellous gift of assimilating information rapidly; and his wide scientific knowledge was never at a loss. He was a constant contributor to many leading technical journals in this country, and frequently reported meetings of the British Association for the Times, the Engineer, etc. Prof. Louis was for many years foreign representative of the International Association of Journalists. He acted as hon. secretary for the congress which was held in this country in the year 1909, when his gifts as a linguist, his knowledge of foreign countries, and his geniality and savoir faire were invaluable. His loss is deeply felt by a large number of friends.
IN an article entitled "Stricken Serbia" in the Times of May 28, a distressing picture is given of the ravages of typhoid, typhus, and relapsing fever in that country. The two last diseases, which accounted for some 15,000 of the sufferers, were traced to the Austrians who left their sick at Valievo. The conditions on the arrival of the various relief organisations were appalling; an observer speaks of "the mass of fever patients lying in all their filthy and verminous rags on the floors and under the beds of what are not hospitals but mere charnel-houses for the dying"! Nevertheless, in less than two months the diseases were got under, typhus being almost stamped out. This was accomplished by the energy of Colonel Hunter and Lieut.-Colonel Stammers and the devoted band of doctors and nurses acting under their direction. Six months of war had depleted the country of stores and provisions, and hospital stores and comforts had to be brought from England or Malta. Clearly the first need was to break the lines of communication between the troops and the rest of the country. Quarantine stations were established, all railway communication was suspended for fifteen days, all leave from the army was stopped, and soldiers on leave were recalled. The problem of typhus was comparatively simple, as this disease is spread by lice. Three weeks were given up to the disinfection of clothes, blankets, and linen; and hospitals and their contents were disinfected. Notification was enforced, and infectious patients were removed to isolation hospitals. The sanitary staff travelled from place to place in a special train and instructed the people what to do. Preventive inoculation especially against cholera was also resorted to, as this dread disease is very likely to break out under such conditions. The work was carried out by two hospital units, Lady Paget's and Lady Wimborne's, each with about fifty doctors, nurses, and orderlies, and by smaller contingents under Dr. Moon and Mrs. Hardy.
MR. G. F. CHAMBERS, whose death on May 24 at seventy-four years of age we regret to record, was a very voluminous writer on many subjects, legal, political, ecclesiastical, but the book which gives him the best scientific claim to remembrance was written when he was not yet twenty years of age. This was the "Handbook of Descriptive Astronomy." In 1861, though England then boasted a distinguished and zealous band of amateur astronomers, amongst whom
were numbered men like Birt, Dawes, De la Rue, Green, and Webb, there was no literature to meet their special necessities. The first edition of Mr. Chambers's "Handbook," published in December, 1861, formed an admirable compendium of the results of astronomical science at that date in the departments which particularly appealed to the amateur observer. The success of the adventure was pronounced; other editions followed, and when the fourth and last edition was brought out, in 1889 and 1890, the book had expanded into three volumes, containing altogether more than 1600 pages. The speciality of the work lies in the number which it contains of useful catalogues, of auxiliary tables for the simple reductions of a private observatory, of descriptions of different forms of telescope mountings and houses adapted for small observatories, and so forth. Mr. Chambers, though he was himself the possessor of one of the small observatories for which he especially catered, did not contribute much to science by his own personal observations, but he was indefatigable in compiling useful or popular works on the subject. One work upon which he bestowed a great amount of labour, viz. his revision of Admiral Smyth's "Celestial Cycle," proved a failure, since his direct practical knowledge of double star astronomy was not sufficient to warrant him in undertaking so important a task; but his smaller and more popular astronomical books have met a cordial reception from the public. These are his "Pictorial Astronomy" and his "Stories" of the sun, stars, eclipses, weather, and comets. He was an original member of the British Astronomical Association, and served for many years as vice-president or on its council. In connection with this association, he took a great interest in eclipse expeditions, and spared no time or energy in ascertaining the best routes by which intending observers could travel to places within the shadow track.
THE Daily Telegraph published a telegram "From our own correspondent at Copenhagen," on May 26, reporting that "A Danish surgeon and scientist of the highest reputation has succeeded in discovering what the German soldiers use to protect themselves against the asphyxiating gases which they employ against the enemy." The "discovery" is that the Germans make use of solutions of hyposulphite and bicarbonate of soda to moisten their respirators. The announcement reminds us, however, of the belated discovery of the lamented death of Queen Anne! The use of such solutions is well known to all workers with chlorine gas, and was mentioned in daily papers a day or two after the Germans commenced to discharge the gas upon our troops.
DR. A. R. FRIEL describes, under the term "piantication," modifications of microbes induced by treating them one or more times with blood-serum, which are transmissible to, and cumulative in, their descendants. Thus organisms which previously almost completely resisted ingestion by leucocytes when "pianticated" are ingested in large numbers by leucocytes (South African Institute for Medical Research, January 26, 1915).
IN the Proceedings of the National Academy of Sciences, Washington (vol. i., p. 256), Dr. C. D. Walcott announces the discovery of bacteria in one of the petrified algæ from pre-Cambrian rocks in Montana, to which reference was made in NATURE last week (p. 354). They appear in the algal tissue as irregular chains of darkly stained cells from 0.95 to 1.3 microns in diameter, and are very suggestive of Micrococcus. It is not always possible to distinguish with certainty such fossils from purely mineral structures, but satisfactory traces of bacteria have already been detected in the fossilised remains both of animal and plant tissues in European Palæozoic rocks, and they are to be expected among the earliest organisms. THROUGH Mr. Bassett Digby, the geological department of the British Museum (Natural History) has lately obtained a well-preserved front horn of the woolly rhinoceros from the frozen earth of northern Siberia. The specimen measures nearly a metre in length, and, though partly cut as usual by the natives who found it, shows the backward curvature of its slender apex and the lateral compression of its characteristic sharp posterior border. The new horn has been mounted, with a hinder horn already in the museum, above one of the exhibited skulls of woolly rhinoceros from Siberia. It is thus possible to realise the unusually large proportions of the horns in this extinct species.
THE question of the pollution of the air of our manufacturing towns has been a serious one for some time, and the report of the Air Pollution Advisory Board of the city of Manchester will prove a valuable document to those seeking to mitigate a serious evil. It appears that the domestic grate is the principal offender and that the modern factory with mechanical stokers is comparatively, if not absolutely, innocent. In many cases, however, the impurities are not due to indifferent stoking, and for these the electrostatic method of precipitation which has proved so successful in America may be recommended. An interesting account of the method and the results of its application will be found in the Electrical Review for May 14. Briefly, the polluted air passes between electrodes maintained at a difference of potential of 100,000 volts, and the particles of carbon, arsenic, potash, or chlorine are carried by the discharge to one of the electrodes. About five kilowatts are necessary to deal with about 30,000 cu. ft. of air per minute, so that the cost is slight. In many of the cases cited the value of the material recovered in a year exceeded the cost of installation and working of the apparatus.
THE Times Engineering Supplement for May 28 touches on a point in works organisation to which too little attention has been given in this country, especially in the case of small- and medium-sized factories. Before the war, our industries suffered severely from German competition; this has been attributed in varying degrees to free trade, lack of technical education, and so forth. As the output of our factories must now be increased, our contemporary asks: Given perfect workmen and entire absence of alcohol, is the management of such a high order that the output is a maximum? Are the pro
prietors, or directors, so wholehearted in their patriotism that they lose no time? Are they so skilled in their respective spheres that they are able to guide their staff and workmen? In large firms the organisation is generally of a high order of efficiency, but it must be confessed that a large number of small firms work from hand-to-mouth in such a manner that output and delivery of orders to a stated time are quite problematic matters. Many of these smaller concerns are in the hands of a family, or financial men, who know nothing of the work being carried on, and whose sole object is to derive as large an income as possible with the minimum of effort. Such firms which are not producing a good average should be visited by skilled managers, and the real cause located. If lack of capital is the cause, the Government should assist; if incompetent management, the offenders should be removed; and if the cause is lack of plant which cannot be immediately rectified, the factory should be closed and the men drafted to a wellorganised concern.
THE director of the Geological Survey of the Union of South Africa asks us to announce that no annual report of the Survey will be issued for the year 1914. The announcement is made to spare the necessity of inquiries from the many scientific institutions, etc., which are on the complimentary list of the Survey.
ERRATUM. In equation 2 on p. 359 of NATURE of May 27, the symbol T should appear as a factor in the denominator of the fraction, and there should be a minus sign before r in the numerator. Both omissions were overlooked by the author in two proofs corrected by him.
OUR ASTRONOMICAL COLUMN. OBSERVATIONS OF NEBULÆ AT THE HELWAN OBSERVATORY.-Mr. H. Knox Shaw describes in Bulletin No. 15 of the Helwan Observatory the observations made with the Reynolds reflector up to the end of September of last year, this paper being a continuation of that published in Bulletin No. 9. This work is described as being somewhat of a reconnaissance to see which nebula would repay_photographing with long exposures when the new Ritchey 30-in. mirror is mounted. The paper gives a list of the nebulæ observed, and also one of thirty-one new nebulæ confirmations of which have been made by second photographs in each case. Referring to N.G.C. 6729, the nebulæ attached to the variable star R. Coronæ, Aust., the author states that this nebula is certainly variable, and the question as to how its variability is connected with that of the star is being studied, and is promised as a separate paper.
STARS WITH PROPER MOTION EXCEEDING 0.50" ANNUALLY.-Mr. Adriaan van Maanen contributes to the April number of the Astrophysical Journal (vol. xli., No. 3) a very useful list of stars with proper motion exceeding 0.50" annually. This list is based on Porter's and Kobold's lists, which indicated proper motions greater than 0.50" annually, on Bossert's catalogue of proper motions of 2641 stars, and on numerous other subsequently published notes on stars of large proper motion. list The present is made as far as possible homogeneous throughout, and deals with 533 stars. A column is given showing the spectra of the stars as determined by Mr. W. S. Adams or Miss Cannon. The positions in right ascension and declination are
for the equinox 1900-0, and the proper motions are given for both amount and direction. Remarks regarding uncertainties in proper motions are added, and in the case of double stars, the number in Burnham's General Catalogue is given. Attention is directed to a list by Innes of proper-motion stars south of 19°; this appeared subsequent to the printing of the present paper, and Mr. van Maanen adds the numbers of Innes's list which should be included in his communication.
ORBITS OF VARIABLE RADIAL VELOCITY STARS.-In the April number of the Journal of the Royal Astronomical Society of Canada some reductions are given of the measures of the variable radial velocities of stars. The orbit of 136 Tauri, deduced by J. B. Cannon, is based on sixty plates taken at the Ottawa Dominion Observatory between November, 1911, and January, 1915. The paper gives a list of the observations. The spectrum of this star is of the A type, and the deduced period is 5.969 days. The same writer deals also with the orbit of Andromeda, a star of the K type spectrum, and utilises fifty-eight plates taken at the same observatory between September, 1913, and February, 1915. On the average about twelve lines were measured on each plate, and, generally speaking, the agreement was very fair. The final period derived was 17.767 days. In both of the above stars comparisons are made with the Lick Observatory results. The orbits of the spectroscopic components of 50 Draconis are discussed by W. E. Harper from the velocities of the ten plates secured at the Yerkes Observatory, and thirty-four taken Dominion Observatory. Both spectra were of the A type. The Yerkes and Ottawa observations both indicate a period of 4120 days. The determination of the orbit of the spectroscopic binary, 1149 Groombridge, was also undertaken by the same writer using thirty spectrograms secured at Ottawa and three secured by Adams. The star is of the A5 type, and the spectrum has numerous lines well adapted for measurement.. In the list of final elements derived the period is given as 9.944 days.
MANCHESTER ASTRONOMICAL SOCIETY.-No. 1 of the Journal of the Manchester Astronomical Society, for the session 1913-14, gives a brief account of the origin of the society and a statement as to its growth since its foundation in 1903. The object is for the association of amateur astronomers, for mutual help, their organisation in the work of astronomical observation, and the encouragement of a popular interest in astronomy. The society numbers at present 131 members, and its president is the Rev. A. L. Cortie, S.J. The journal gives excellent portraits of the present and past four presidents; and the address of the president on the origin of the sun and stars is printed and illustrated with three plates. A paper on Japanese and other magic mirrors is from the pen of the late Mr. T. Thorp, who was an original and active member of the society. Lunar photography, by Mr. William Porthouse; astronomy and æsthetics, by Mr. E. Denton Sherlock; and a remarkable solar prominence, by Mr. A. Buss, form the subjects of other papers printed in this issue.
A. Chaston Chapman, the chairman, in opening the discussion, pointed out that from the technical point of view the purposes to be served by such analyses were, first, to indicate the general character of the process by which any particular extract had been prepared; secondly, to throw some light on the source of the extract and its genuineness or otherwise; and, finally, to furnish information as to the physiological properties of dietetic value. He then gave an outline of the existing methods of analysis, and emphasised their limitations; more particularly the practice of returning the residual nitrogen as "meat bases," using the factor 3.12 for the conversion was a source of uncertainty and confusion, especially as Hehner had suggested the use of the ordinary protein factor, 6-25, for the same purpose. The best plan was to return the actual nitrogen percentages.
Dr. F. Gowland Hopkins, dealing with the question of the food value of meat extracts and similar preparations, pointed out that the animal body dealt not with the intact proteins, or even with the albumoses and peptones, but with the free amino-acids which were the individual constituents of the protein molecules. The way in which these acids were grouped in the protein molecule was not of much consequence, but the effects produced by the individual amino-acids were of extreme importance. He described physiological experiments which he had made showing that when rats were given a diet including a complete amino-acid mixture corresponding with the proteins of an ordinary diet, the growth was almost exactly normal, but when arginine and histidine were removed from the mixture, growth ceased immediately, but was again resumed when the missing constituents were added. The removal of tryptophane produced similar results, and Osborne and Mendel had, in America, shown that cystine was similarly essential. It did not follow that this was the case with every amino-acid, and the question as to which of these were vitally necessary offered a large field for investigation. Experiment had shown that in the case of rats, the critical minimum for arginine lay somewhere between 2 and 1 per cent. The functions of the individual amino-acids were not confined merely to flesh formation. Thus, for example, the effect of feeding animals on zein, which was deficient in both tryptophane and lysine, was not only to restrict growth, but to shorten the survival; the same was true with zein plus lysine, but with zein plus tryptophane the animal was able to maintain its weight for a long period, although it did not grow.
Dr. E. P. Cathcart said that observations at present available were so scanty that it could not be said with certainty that creatine and creatinine had a special niche in the organism. He did not think that any end would be gained by the separate estimation of these two substances. Mr. A. R. Tankard and Mr. E. Hinks dealt with questions of procedure, and Dr. Percival Hartley described his experience of Van Slyke's method of estimating amino-nitrogen; further remarks were made by Dr. Rideal, Prof. Barger, Dr. Harden, and Dr. S. Walpole.
MUSEUMS AND EDUCATION.
HE Museums Journal for May contains an interesting and suggestive article on the educational work of American museums, by the director of the Charleston Museum. It is abundantly clear from this that the functions of the museum in America are, so to speak, intensively cultivated. And nowhere is this more apparent than in the facts which he gives in regard to the co-operation which has grown up between the museums and the public schools. It is now the rule, he tells us, for children to be brought
in batches by their teachers for regular instruction by the museum staff. This is given partly in the form of tours round the galleries, and partly by lectures given in class-rooms set apart for this purpose. Instruction is also provided for higher-grade scholars and for the teachers themselves.
In some places, as in Philadelphia, to obviate loss of time in taking large classes to the museum, it is now customary to send travelling exhibits from the museum to the schools.
Further efforts to utilise the museum as a means of education have resulted in the formation of children's museums, where collections of birds and beasts likely to interest children are arranged so as to convey some definite and easily assimilated ideas, as, for example, on the significance of animal coloration, or the shapes of animals.
The rapid increase of vocational and industrial training in the public schools of America, the author remarks, is already creating a demand for further assistance from museums, which they are meeting by lectures on the relation of natural resources to commerce and industry, and by exhibits illustrating processes of manufacture. It is probable that this is but the beginning of a phase of museum work which will ultimately occupy a large place in the activities of general museums and lead to the establishment of special industrial museums.
This appreciation of the functions of museums is a healthy sign, and might well be emulated in this country, where the museum is still commonly looked upon as a kind of "curiosity shop." Isolated attempts have been made on the part of many of our local museums to induce the public to make greater use of the collections exhibited for their benefit, but the Board of Education has yet to be educated in regard to the possibilities of museums. At the British
Museum much valuable work is being done in the direction of economic zoology, but the scope of this work is hampered by lack of adequate financial support.
This very suggestive essay should be widely read in this country, for there can be no doubt that the educational advantages offered by our museums are little known or utilised by educational authorities.
MEASUREMENT OF THE DISTANCES OF THE STARS.1
OR the lecture in honour and memory of Edward Halley, which it is my privilege to deliver this year, I have chosen an account of the persistent efforts made by astronomers to measure the distances of the fixed stars. For many generations their attempts were unsuccessful, though some of them led to great and unexpected discoveries. It is less than eighty years ago that the distances of two or three of the nearest stars were determined with any certainty. The number was added to, slowly at first, but afterwards at a greater rate, and now that large telescopes are available and photographic methods have been developed, we may expect that in the next few years very rapid progress will be made.
For many centuries astronomers had speculated on the distances of the stars. The Greeks measured the distance of the moon; they knew that the sun and planets were much further away, and placed them correctly in order of distance, guessing that the sun was nearer than Jupiter because it went round the sky in one year while Jupiter took twelve. The stars, from their absolute constancy of relative position, were rightly judged to be still more distantbut how much more they had no means of telling. In 1543 Copernicus published "De Revolutionibus 1 The "Halley Lecture (slightly abridged), delivered at Oxford on May 20, by Sir F. W. Dyson, F.R.S., Astronomer Royal,
Orbium Cœlestium," and showed that the remarkable movements of the planets among the stars were much easier to understand on the hypothesis that the earth moved annually round the sun. Galileo's telescope added such cogent arguments that the Copernican system was firmly established. Among other difficulties which were not cleared up at the time one of the most important was this: If the earth describes a great orbit round the sun, its position changes very greatly. The question was rightly asked: Why do not the nearer stars change their positions relatively to the more distant ones? There was only one answer. Because they are so extremely distant. This was a hard saying, and the only reply which Kepler, who was a convinced believer in the earth's movement round the sun, could make to critics was "Bolus erat devorandus."
Although no differences in the positions of the stars were discernible to the naked eye, it might be that smaller differences existed which could be detected by refined astronomical measurements. To the naked eye a change in the angle between neighbouring stars not more than the apparent diameter of the sun or moon should be observable. No such changes are perceived. The stars are-it may be concluded-at least two hundred times as distant as the sun. With the instruments in use in the seventeenth centurybefore the telescope was used for the accurate measurement of angles-angles one-twentieth as large were measurable, and the conclusion was reached that the stars were at least four thousand times as distant as the sun. But no positive results were obtained. Attempts followed with the telescope and were equally unsuccessful. Hooke tried to find changes in the position of the star y Draconis and failed. Flamsteed, Picard, and Cassini made extensive observations to detect changes in the position of the pole star and failed. Horrebow thought he had detected slight changes in the position of Sirius due to its nearness in a series of observations made by Römer. He published a pamphlet, entitled "Copernicus triumphans," in 1727, but the changes in the position of Sirius were not verified by other observers, and were due to slight movements of Römer's instru
Thus in Halley's time it was fairly well established that the stars were at least 20,000 or 30,000 times as distant as the sun. Halley did not succeed in finding their range, but he made an important discovery which showed that three of the stars were at sensible distances. In 1718 he contributed to the Royal Society a paper entitled "Considerations on the Change of the Latitude of Some of the Principal Bright Stars." While pursuing researches on another subject, he found that the three bright stars-Aldebaran, Sirius, and Arcturus-occupied positions among the other stars differing considerably from those assigned to them in the Almagest of Ptolemy. He showed that the possibility of an error in the transcription of the manuscript could be safely excluded, and that the southward movement of these stars to the extent of 37', 42', and 33'-i.e. angles larger than the apparent diameter of the sun in the sky-were established. He remarks:
'What shall we say then? It is scarce possible that the antients could be deceived in so plain a matter, three observers confirming each other. Again these stars being the most conspicuous in heaven are in all probability nearest to the earth, and if they have any particular motion of their own, it is most likely to be perceived in them, which in so long a time as 1800 years may show itself by an alteration of their places, though it be utterly imperceptible in a single century of years."
This is the first good evidence, i.e. evidence which
we now know to be true, that the so-called fixed stars are not fixed relatively to one another. It is the first positive proof that the distances of the stars are sensibly less than infinite. This, then, is the stage at which astronomers had arrived less than two hundred years ago. The stars are at least 20,000 or 30,000 times as distant as the sun, but three of the brightest of them are perceived to be not infinitely distant.
The greatest step in the determination of stellar distances was made by another Oxford astronomer, James Bradley. His unparalleled skill as an astronomer was early recognised by Halley, who tells how Dr. Pound and his nephew, Mr. Bradley, did, myself being present, in the last opposition of the sun and Mars this way demonstrate the extreme minuteness of the sun's parallax, and that it was not more than 12 seconds nor less than 9 seconds." Translated from astronomical language, the distance of the sun is between 95 and 125 millions of miles. Actually the distance is 93 million miles. The astronomer who so readily measured the distance of the sun entered on the great research which had baffled his predecessors-the distance of the stars.
The theory of the determination of stellar parallax is very simple: the whole difficulty lies in its execution, because the angles are so small that the slightest errors vitiate the results completely. Even at the present time with large telescopes, and mechanism which moves the telescope so that the diurnal movement of the stars is followed and they appear fixed to the observer in the field of the telescope, and with the additional help of photography, the determination of the parallax of a star requires a good deal of care, and is a matter of great delicacy. But in Bradley's time telescopes were imperfect, and the mechanism for moving them uniformly to follow the diurnal rotation of the stars had not been devised.
This was in some ways very fortunate, as the method Bradley was forced to adopt led to two most important and unexpected discoveries. Every day, owing to the earth's rotation, the stars appear to describe circles in the sky. They reach the highest point when they cross the meridian or vertical plane running north and south. If we leave out all disturbing causes and suppose the earth's axis is quite fixed in direction, a star S, if at a great distance from the earth, will always cross the meridian at the same point S; but, if it is very near, its movement in the small parallactic ellipse will at one period of the year bring it rather north of its mean position and at the opposite period an equal amount south.
Bradley, therefore, designed an instrument for measuring the angular distance from the zenith, at which a certain star, y Draconis, crossed the meridian. This instrument is called a zenith sector, and is shown in the slide. The direction of the vertical is given by a plumb-line, and he measured from day to day the angular distance of the star from the direction of the vertical. From December, 1725, to March, 1726, the star gradually moved further south; then it remained stationary for a little time; then moved northwards until, by the middle of June, it was in the same position as in December. It continued to move northwards until the beginning of September, then turned again and reached its old position in December. The movement was very regular and evidently not due to any errors in Bradley's observations. But it was most unexpected. The effect of parallax-which Bradley was looking for-would have brought the star furthest south in December, not in March. The times were all three months wrong. Bradley examined other stars, thinking first that this might be due to a movement of the earth's pole. But this would not explain the phenomena. The true explanation, it is said, although I do not know how
truly, occurred to Bradley when he was sailing on the Thames, and noticed that the direction of the wind, as indicated by a vane on the mast-head, varied slightly with the course on which the boat was sailing. An account of the observations in the form of a letter from Bradley to Halley is published in the Philosophical Transactions for December, 1728:
"When the Year was compleated, I began examine and compare my Observations, and having pretty well satisfied myself as to the general Laws of the Phenomena, I then endeavoured to find out the Cause of them. I was already convinced that the apparent Motion of the Stars was not owing to a Nutation of the Earth's Axis. The next thing that offered itself, was an Alteration in the Direction of the Plumb-line, with which the instrument was constantly rectified; but this upon trial proved insufficient. Then I considered what Refraction might do, but here also nothing satisfactory occurred. At last I conjec tured that all the Phenomena hitherto mentioned, proceeded from the progressive Motion of Light and the Earth's Annual Motion in its Orbit. For I perceived that, if Light was propagated in Time, the apparent Place of a fixed Object would not be the same when the Eye is at Rest, as when it is moving in any other Direction, than that of the Line passing through the Eye and the Object; and that, when the Eye is moving in different Directions, the apparent Place of the Object would be different."
This wonderful discovery of the Aberration of Light is usually elucidated by the very homely illustration of how an umbrella is held in a shower of rain. Suppose the rain were falling straight down and a man walking round a circular track: he always holds the umbrella a little in front of him-because when he is walking northward the rain appears to come a little from the north, when he is going eastward it appears to come a little from the east, and so on.
Although the phenomena Bradley had observed were almost wholly explained in this way, there were still some residual changes, which took nineteen years to unravel; and he explained these by a nutation or small oscillation of the earth's axis, which took nineteen years to complete its period. I cannot dwell on these two great discoveries. For our present purpose, it should be said that aberration and nutation cause far greater changes in the apparent positions of the stars than, we now know, are caused by parallax. Until they were understood and allowed for or eliminated, all search for parallax must have been in vain. Further, Bradley's observations showed that in the case of y Draconis, at any rate, parallax did not displace the star by so much as 10" from its mean position, or that the star was 200,000 times as distant as the sun. We may say that Bradley reached to just about the inside limit of the distances of the
Let me now try to give some idea of what is meant by a parallax of 1", which corresponds to a distance 200,000 times that of the sun. Probably many of you have looked at the second star in the tail of the Great Bear, Mizar, it is named, and have seen there is a fainter star near it, which you can see nicely on a fine night. These stars are 600" apart; with a big telescope with a magnification of 600 times-and this is about as high a magnification as can be generally used in England-two stars 1" apart are seen double just as clearly as Alcor and Mizar are seen with the naked eye. I think this is the most useful way to think of 1"-a very small angle, which one needs a magnification of 600 times to see easily and clearly. Bradley showed that. y Draconis did not wander by this amount from its mean position among the stars in consequence of our changing view-point.
The next attempt to which I wish to refer is the