February 25, 191 5] 



NATURE 



'o\ 



extreme red in another, if the colour character of 

 the detail being searched for is unknown. 



By modifying- such extreme measures as just 

 described, it is obviously possible by photography 

 to intensify or diminish the apparent effect of any 

 colour, and so, as for example in stained physio- 

 logical sections, to improve on the original, or 

 from one original to get two (or more) photographs 

 that represent the section in, as it were, two (or 

 more) different conditions as to staining. 



The photographer has also by such means a 

 very large measure of control over the effect of 

 mist. The writer has shown that light is scattered 

 by particles of a diameter equal to half a wave- 

 length of the scattered light. By passing a 

 brilliant pencil of light through a flask of filtered 

 distilled water the path of the pencil may be 

 scarcely visible (the "Tyndall effect"). But by 

 photographing the arrangement on an ordinary 

 plate, that is, using blue and ultra-violet light, the 

 path of the light will probably be shown very con- 

 spicuously indeed, the particles that escaped re- 

 moval by filtration being not large enough to 

 scatter light of greater wave-length than the 

 ultra-violet and the extreme blue. When the 

 particles of the mist or turbidity are graduated 

 in size, as they generally are, and are not coarse 

 Uke the dust that may be stirred up from a dry 

 road, then the longer the wave-lenglh of the light 

 used the less does the mistiness show in the photo- 

 graph. When the air is misty, by the use of a 

 yellow filter distant objects may be photographed 

 more clearly than they can be seen, and by using 

 only infra-red light Prof. Wood has obtained 

 photographs in which the sun shines, though there 

 was only a grey cloudiness visible. 



Though the colour sensitising of plates is now 

 as much the manufacturer's business as the pre- 

 paration of emulsions and the coating of the plates 

 therewith, the experimentalist may wish to try the 

 effect of a dye for himself. The solution must be 

 very weak indeed, say one of the dye to from 

 10,000 to 50,000 times its weight of water. 

 Ammonia is often advantageous to the extent of 

 one per cent, of the whole. The best formula is 

 simply a matter of experiment. The following, 

 for example, is a varation recently published by 

 Messrs. Michaud and Tristan (Brii. Jnl. of Phot., 

 Ixii., 56) for sensitising for the red and infra- 

 red : — ■ 



Alcohol 50 per cent., 200 c.c. 



Ammonia, 4 c.c. 



Alizarin blue S, 004 gram. 



Silver nitrate 10 per cent, solution, 5 drops. 

 The same dye is efficient without the silver salt 

 and the alcohol, but presumably these are advan- 

 tageous. In general, after the plate has been in 

 the dye solution for two or three minutes, it is 

 washed for a few minutes and then dried. Plates 

 so treated vary very much as to the time that they 

 will remain in good condition. For example, 

 plates bathed in the solution quoted above must be 

 used within a few hours, but in other cases they 

 may last for months. 



NO. 2365, VOL. 94] 



It is much less risky to make colour filters tha« 

 to bathe plates, as most dye solutions will be 

 absorbed by a clean gelatine film. But colour 

 filters are now made by several firms, and the 

 Wratten department of Kodak catalogues stand- 

 ardised colour filters of nearly a hundred different 

 transmissions. CHAP.^f.\^■ Jones. 



PROF. G. F. J. ARTHUR AUWERS. 

 T HE brief paragraph in our issue of last week, 

 ■*• announcing the death of Prof. Auwers, 

 must have been read with the deep regret that 

 follows the loss of one who long occupied a com- 

 manding position in the world of science, and 

 whose place it will be difficult to fill. For more 

 than fifty years he had illuminated the science of 

 astronomy, and by providing much of the material 

 by which it is hoped to attack successfully the 

 problem of the structure of the sidereal universe, 

 he linked together the astronomical thoughts and 

 methods of the past with the philosophical prob- 

 lems that engross and captivate the attention of 

 astronomers of to-day. 



In the history of astronomy of position, by 

 which is understood the accumulation and arrange- 

 ment of facts depending on a star's place in the 

 sky, three names stand out prominently. Bradley, 

 in whose valuable series of obser\'ations, long- 

 waiting for an interpreter, lay hidden the secret 

 of stellar proper motion ; Bessel, who made these 

 measures available to the astronomers of his day ; 

 and Auwers, whose early appreciation of the 

 necessity of the highest accuracy gave to these 

 observations an increased value by his long and 

 patient examination, laying the foundation of 

 that system of thorough uniformity which has 

 welded meridian observations into a more con- 

 sistent whole, facilitating the combination of star 

 catalogued places on a common basis, by the 

 removal of systematic errors or discrepancies. 

 Auwers taught the necessity for a higher standard 

 of accuracy, and it is not too much to say that 

 in the department of reduction and discussion of 

 observations he long stood without a rival. His 

 forte lav in the control and management of large 

 masses of work, in the unhurried, careful super- 

 vision of every stage, bringing an acute and 

 trained judgment to bear equally on all parts of 

 the investigation. 



If his reputation rests especially on his re-reduc- 

 tion of Bradley, it must be remembered that he 

 encouraged and assisted other large undertakings. 

 He took a prominent part in the re-observation 

 of the Durchmusterung zones, a work of many 

 years' international co-operation successfully 

 carried out under the auspices of the Astrono- 

 mische Gesellschaft. He was among the first to 

 investigate the proper motion of faint stars, and 

 he foreshadowed some of the conclusions that 

 have been established by the most modern and 

 thorough of inquiries. The determination of solar 

 parallax by the method of the transit of V'enus 

 is a somewhat discredited problem now, but forty 



