1882.] On the Cause of the Light Border in Photographs, SfC 63 



V. " On the Cause of the Light Border frequently noticed in 

 Photographs just outside the Outline of a Dark Body seen 

 against the Sky; with some Introductory Remarks on 

 Phosphorescence." By Professor G. G. Stokes, Sec. U.S. 

 Received May 20, 1882. 



An observation I made the other day with solar phospkori, though 

 not involving anything new in principle, suggested to me an explana- 

 tion of the above phenomenon which seems to me very likely to be 

 the true one. On inquiring from Captain Abney whether it had 

 already been explained, he wrote : " The usual explanation of the phe- 

 nomenon 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 part strongly acted upon 

 by light." As this explanation seems to me to involve some difficul- 

 ties, I venture to offer another. 



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

 wholly uninteresting on its own account, as affording a pretty illus- 

 tration 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,* of considerable aperture for its focal length. 

 A phosphorus is taken, suppose sulphide of calcium giving out a deep 

 blue light,t 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 excite- 

 ment of the phosphorus has been refreshed. This is separated by a 

 dark halo from the general ground, which shines by virtue of the 



* 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. 



f The experiments were actually made, partly with a tablet painted with Bal- 

 main's luminous paint, partly with sulphide of calcium and other phosphori in 

 powder. 



