November 28, 1895] 



NATURE 



inferred from the most subtle differences of light and shade in 

 the colours of objects, in association with previous experience 

 derived through other senses. The picture is always an optical 

 illusion ; and this additional illusiveness conferred .on the photo- 

 L^raph by the method invented by Maxwell on the basis of the 

 three-colour theory of vision, is surely a strong confirmation of 

 that theory. 



These results are attained by no new photographic opera- 

 tions. It is necessary to use good orthochromatic plates sensitised 

 into the red, and also to have affixed in the lens an orthochro- 

 matic screen cutting of the ultra-violet light in the usual manner. 

 I'he exposure is somewhat longer than the ordinary exjxjsure, 

 fc.i we can of course only use visible light, and of this a part is 

 stopped by the taking screen. The ordinary backs may be used. 

 The displacement of the sensitive film from accurate register 

 with the ground glass camera screen, owing to the presence of 

 the taking screen in front of it, may be corrected (if thought 

 necessarj) by simply reversing the surface of the ground glass 

 camera screen, turning the muffled side outward. This .secures 

 that the image will be accurately focussed in the plane of the 

 sensitive surface. Negatives and positives may be used as 

 ordinary negatives or positives till it is desired to recall the 

 original colours. Thus, for those who wander with the camera, 

 the possession of but the one seeing screen to test results is 

 sufficient, and of course the one taking screen suffices to take an 

 indefinite number of plates. 



These considerations lead us naturally to observe that the 

 registration of colour being really carried in the silver image, 

 which with very little care in manipulation may be made 

 permanent, secures that the colours are permanent. A faded 

 screen may at any time be made good by a fresh screen ; the 

 colours in all cases being spectroscopically chosen, we are assured 

 of the reproduction of the original colour. In this aspect the 

 necessity of the detached colour screen is no disadvantage, but 

 rather a necessary safeguard against the inevitable fading attend- 

 ing most pigment colours. 



D^ 



COMET MAGNITUDES. 



|R. HOLETSCHEK, of the Vienna Observatory, has re- 

 cently communicated to the Imperial Academy of Sciences 

 a paper on the magnitude and brilliancy of comets and their 

 tails, with the view of arranging them in "magnitudes" or 

 orders of brilliancy in a manner similar to that in which stars 

 are arranged according to their lucidity. Further, from the data 

 given as to the apparent length of the tail, the true length of the 

 tail has been computed, and an inquiry instituted as to the pos- 

 sibility of tail formation and its probable length, based on the 

 resulting magnitude of the comet and its perihelion distance. 



If it be true that the brilliancy of a comet varies as the squares 

 of the distances from the sun (r), and from the earth (A), then 

 from observations made at various points of the orbit, the same 

 " magnitude" ought to result for the values r = A = i. This 

 magnitude Dr. Holetschek has deduced, where sufficient data 

 existed, and the results can be practically arranged m two classes : 

 one, in which the deduced magnitudes derived from various values 

 of r and A so nearly agree that a mean can be taken ; the other, 

 in which is shown a regular progress, and always in the direction 

 that the deduced magnitudes with small radii vectures, therefore 

 when the comet is near perihelion, are greater than when at 

 large distances from the sun. The origin of this is due to the 

 fact that the second power of the radius vector does not fully 

 represent the variations occurring in comets as they approach 

 the sun, at which time their brilliancy is more increased than is 

 shown by the ratio i : r^t^. The first case, it is suggested, is 

 only a special case of the second, arising through insufficiency 

 of description. This is most clearly shown when the time 

 covered by the observations is short, or the details so wanting in 

 accuracy that the deviation from the ratio i : r^A'^ cannot be 

 IsHown with certainty. The conclusion drawn is that the formula 

 so generally used can represent the brilliancy of a comet at 

 (lifTerent distances from the sun for a short time only, and is in- 

 applicable for long periods. 



Dr. Holetschek uses the deduced magnitude in the neighbour- 

 hood of the perihelion, valuable as showing the greatest bril- 

 liancy attainable in a particular comet, as the data for forming 

 the comets into orders of magnitude, and inquires how far the tail 

 formation is connected with this magnitude and the perihelion 

 distance. He decides from his material, that when the deduced 



NO. I 36 1, VOL. 53] 



magnitude is 6 or lower than 6, only a short and feeble tail, or 

 one not visible to the naked eye, is possible. Comets with a de- - 

 duced magnitude of 4, or still brighter, have a tail well visible, 

 which is the greater, the smaller the perihelion distance, and the 

 smaller, the greater this distance. Within the limits between 4 

 and 5 magnitude, if we exclude very great perihelion distances, 

 lie the ]X)Ssibilities of a considerable tail development. 



Dr. Holetschek has also considered the diameters of comets, 

 and sought to introduce order by reducing the apparent diameter 

 to that corresponding to a distance of the comet from the earth 

 equal to unity. In the case of Halley's comet, no diminution or 

 variability is to be detected in either its brilliancy or the length 

 of its tail. The same values serve from 1456 to 1835. As, 

 however, on account of the continual development of the tail, a 

 diminution of the mass is probable, it cannot be decided 

 whether the approximate constancy, shown in the investigation, 

 arises from inadequacy in the ol)servations themselves, or is 

 produced by certain processes existing in cometary bodies. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 



Oxford. — The delegates of the Common University Fund 

 have elected Mr. Edwin Stephen Goodrich, Merton College, to 

 the Biological Scholarship at Naples for the year 1895-96. 



Cambridge. — The report to the Senate of the Engineering 

 Laboratory Syndicate, dated November 9, 1895, is a ver)- 

 satisfactory record of energetic work. The University was 

 unable to provide more than ^1000 for the building and equip- 

 ment of the laborator}', which was estimated to cost about 

 2"6ooo. By steady and enthusiastic efforts Prof. Ewing and his 

 colleagues succeeded in collecting from many sources, within and 

 without the University, the ;,{^ 5000 that were needed ; and what is 

 still more to their credit, have completed the work in hand with 

 a small balance to the good. The continued growth of the 

 department, in which there are now eighty-five students under 

 instruction, makes a further extension of the buildings urgently 

 necessary. A site has been provided for the purpose, but further 

 funds are needed for construction. The workshops, never 

 intended to be other than temporary, must soon be rebuilt, and 

 more lecture-room accommodation must be provided. The 

 department has certainly justified its existence, and the Syndicate 

 have proved themselves to be worthy stewards of the funds 

 placed at their disposal. It is to be hoped that, with these 

 guarantees that they will be well used, the needful moneys may 

 ere long be forthcoming. The valuable services of Mr. Dalby 

 and Mr. Lamb, the demonstrators of mechanism and engineering, 

 in the work of organising the laboratory', are mentioned with 

 cordial appreciation in the report. Among the donors of con- 

 tributions of over ;^ioo are the Duke of Devonshire, the late 

 Earl of Derby, Mr. Frank McClean, and Dr. John Hopkinson, 

 and eleven benefactors have given £i<X) each. Valuable 

 donations of apparatus and of books have helped greatly towards 

 the furnishing of the laboratory. 



The amount, clear of all expenses, available for the Robertson 

 Smith memorial, is ;ff 1450. It has been agreed by the sub- 

 scribers that sufficient of this amount should be invested to pro- 

 duce an annual income of ^^30, such income to be employed on 

 the continuance and extension of Prof. Robertson Smith's 

 library, which he bequeathed to Christ's College. It was also 

 resolved that the remainder of the amount collected, after all 

 expenses have been paid, be handed over to the University for 

 the purchase of Oriental MSS. for the University Library, which 

 shall be marked as having l)een acquired by means of the fund. 

 It is estimated that about £Tpo will be handed over to the Syndics 

 of the Library. 



It is reported that Mr. P. N. Russell has given the sum of 

 ;^50,ooo to endow a school of engineering in connection with 

 the Sydney University. 



An address on the present state and position of technical in- 

 struction in this country, delivered by Major-General Sir John 

 Donnelly before the Society of Arts on Wednesday, November 

 20, is printed in full in the current Journal of the Society, 



The annual meeting of the National Association for the Pro- 

 motion of Technical and Secondary Education, and the Con- 

 ference of Representatives of Technical Education Committees, 

 will be held on Tuesday, December 10, at the Royal United 

 Service Institution, Whitehall. 



