142 



NATURE 



[June 8, 



tirely unsecured to the University, and the progress of the 

 department under Mr. Balfour's direction remained liable to 

 sudden check. Upon this representation the Senate unani- 

 mously concurred in the report of the Council and established 

 the professorship. 



Edinburgh. — Dr. James Geikie, F.R.S., has been appointed 

 to the Murchison Chair of Geology and Mineralogy in succession 

 to Frof . Archibald Geikie, Director of the Geological Survey. 

 Dr. Geikie will not enter upon the duties of his class till 

 November next. 



Dr. P. Phillips Bedson, F.C.S., Demonstrator and assistant 

 Lecturer on Chemistry in the Victoria University, Owens Col- 

 lege, has been elected to the Professor.-hip of Chemistry in 

 the Durham University College of Physical Science, Newcastle- 

 on-Tyne. 



SOCIETIES AND ACADEMIES 

 London 



Royal Society, May 25. — "On the Cause of the Light 

 Border frequently no:iced in Photographs just outside the Out- 

 line of a Dark Body seen against the Sky ; with some Intro- 

 ductory Remarks on Phosphorescence." By Prof. G. G. Stokes, 

 Sec. U.S. 



An observation I made the other day with solar phosphori, 

 though not involving anything new in principle, suggested to me 

 an explanation of the above phenomenon which seems to me 

 very likely to be the true one. On inquiring from Capt. Abney 

 whether it had already been explained, he wrote : "The usual 

 explanation of the phenomenon you describe is that the silver 

 solution on the part of the plate on which the dark objects fall 

 has nowhere to deposit, and hence the metallic silver is deposited 

 to the nearest par' strongly acted upon by light." As this 

 explanation seems to me to involve some difficulties, I venture to 

 offer another. 



1. I will first mention the suggestive experiment, which is not 

 wholly uninteresting on its on n account, as affording a pretty 

 illustration of what is already known, and furnishing an easy and 

 rapid mode of determining in a rough way the character of the 

 absorption of media for rays of low refrangibility. 



The sun's light is reflected horizontally into a darkened room, 

 and passed through a lens, 1 of considerable aperture for its focal 

 length. A phosphorus is taken, suppose sulphide of calcium 

 giving out a deep blue light,'- and a position chosen for it which 

 may be varied at pleasure, but which I will suppose to be nearer 

 to the lens than its principal focus, at a place where a section of 

 the pencil passing through the lens by a plane perpendicular to 

 its axis shows the caustic surface well developed. A screen is 

 then placed to intercept the pencil passing through the lens, and 

 the phosphorus is exposed to sunlight or diffuse daylight, so as 

 to be uniformly luminous, and is then placed in position ; the 

 screen is then removed for a very short time and then replaced, 

 and the effect on the phosphorus is observed. 



Under the circumstances described there is seen a circular disk 

 of blue light, much brighter than the general ground, where the 

 excitement of the phosphorus has been refreshed. This is sepa- 

 rated by a dark halo from the general ground, which shines by 

 virtue of the original excitement, not having been touched by the 

 rays which came through the lens. 



2. The halo is due to the action of the less refrangible rays, 

 which, as is well known, discharge the phosphorescence. Their 

 first effect, as is also known, is, however, to cause the phosphorus 

 to give out light ; and if the exposure were very brief, or else the 

 intensity of the discharging rays were sufficiently reduced, the 

 part where they acted was seen to glow with a greeni-h light, 

 which faded much more rapidly than the deep blue, so that after 

 a short time it became relatively dark. 



3. This change of colour of the phosphorescent light can 

 hardly fail to have been noticed, but I have not seen mention of 

 it. In this respect the effect of the admission of the discharging 

 rays is quite different from that of warming the phosphorus, 

 which, as is known, causes the phosphorus to be brighter for a 

 time, and then to cease phosphorescing till it is excited afresh. 

 The difference is one which it seems important to bear in mind 



1 The lens actually used was one of crown glass which I happened to 

 have ; a lens of flint glass would have been better, as giving more separation 

 of the caustic surfaces for the different colours. 



2 The experiments were actually made, partly with a tablet painted with 

 Balmain's fuminous paint, partly with sulphide of calcium and other phos- 

 phori in powder. 



in relation to theory. Warming the phosphorus seems to set the 

 molecules more free to execute vibrations of the same character 

 as those produced by the action of the lays of high refrangi- 

 bility. But the action of the discharging rays changes the 

 character of the molecular vibrations, converting them into others 

 having on the whole a lower refrangibility, and being much less 

 lasting. 



4. Accordingly when the phosphorus is acted on simultaneously 

 by light containing rays of various refrangibilities, the tint of the 

 resulting phosphorescence, and its more or less lasting character, 

 depend materially upon the proportion between the exciting and 

 dischaiging rays emanating from the source of light. Thus day- 

 light gives a bluer and more lasting phosphorescence than gas- 

 light or lamplight. 1 took a tablet which had been exposed to 

 the evening light, and had got rather faint, and, covering half of 

 it with a book, I exposed the other half 'o gaslight. On carrying 

 it intj the dark, the freshly exposed half was seen to be much 

 the brighter, the light being, however, whitish, but after some 

 considerable time the unexposed half was the brighter of the 

 two. 



Again, on exposing a tablet, in one part covered with a glass 

 vessel containing a solution of ammonio-sulphate of copper, to 

 the radiation from a gas flame, the covered part was seen to be 

 decidedly bluer than the rest, the phosphoresce ,ce of which was 

 whitish. Toe former pirt, usually brighter at first than the rest, 

 was sure to be so after a very little time. The reason of this is 

 plain after what precedes. 



A solution of chromate of potash is particularly well suited 

 for a ray filter when the object is to discharge the phosphores- 

 cence of sulphide of calcium. When it stops the exciting rays, 

 it is transparent for nearly the whole of the discharging rays. 

 The phosphorescence is accordingly a good deal more quickly 

 discharged under such a solution than under red glass, which, 

 along with the exciting rays, absorbs also a much larger propor- 

 tion than the chromate of the discharging rays. 



5. I will mention only one instance of the application of this 

 arrangement to the study of absorption. On placing before 

 excited sulphide of calcium a plate of ebonite given me by Mr. 

 Preece as a specimen of the transparent kind for certain rays of 

 low refrangibility, and then removing the intercepting screen 

 from the lens, the transmission of a radiation through the ebonite 

 was immediately shown by the production of the greenish light 

 above-mentioned. Of course, after a sufficient time the part 

 acted on became dark. 



6. I will mention two more observations, as leading on to the 

 explanation of the photographic phenomenon which I have to 

 suggest. 



In a dark room, an image of the flame of a paraffin lamp was 

 thrown by a lens on to a phosphorescent tablet. On intercept- 

 ing the incident rays after no great exposure of the tablet, the 

 place of the image was naturally seen to be luminous, with a 

 bluish light. On forming in a similar manner an image of an 

 aperture in the window shutter, illuminated by the light of an 

 overcast sky reflected horizontally by a looking-glass outside, 

 this image of course was luminous ; it was brighter than the 

 other. On now allowing both lights to act simultaneously on 

 the tablet, the image of the flame being arranged to fall in the 

 middle of the larger image of the aperture, and after a suitable 

 exposure cutting off both lights simultaneously, the place of the 

 image of the aperture on which the image of the lamp had 

 fallen was seen to be less luminous than the remainder, v hich 

 had been excited by daylight alone. The reason is plain. The 

 proportion of rays of lower to rays of higher refrangibility is 

 much greater in lamplight than in the light of the sky ; so that 

 the addition of the lamplight did more harm by the action of 

 the discharging rays which it contained on the phosphorescence 

 produced by the daylight, than it could do good by its own 

 contribution to the phosphorescence. 



7. The other observation was as follows : — The same tablet 

 was laid horizontally on a lawn on a bright day towards evening, 

 when the sun was moderately low, and a pole was stuck in the 

 grass in front of it, so as to cast a shadow on the tablet. After 

 a brief exposure, the tablet was covered with a dark cloth, and 

 carried into a dark room for examination. 



It was found that the place of the shadow was brighter than 

 the general ground, and also a deeper blue. For a short dis- 

 tance on both sides of the shadow the phosphorescence was a 

 little feebler than at a greater distance. 



This shows that, though the direct rays of the sun by them- 

 selves alone would have strongly excited the phosphorus, yet 



