146 Proceedings of the Royal Society of Edinburgh. [Sess. 
XIII. — The Reflective Power of Pigments in the Ultraviolet. By 
Charles Cochrane, M.A., B.Sc., Assistant to the Professor of 
Natural Philosophy in the University of Glasgow. Communicated 
by Dr R. A. Houstoun. 
(MS, received January 25, 1915. Read March 15, 1915.) 
The eye is sensitive only to light of wave-length 7600 to 4000 A.U. With 
the ordinary dry photographic plate and glass lenses we can get an effect 
down to wave-length 3300 A.U. ; with the same plate and quartz lenses we 
can get an effect from a wave-length as short as 2000 A.U. The gelatine 
of the plate absorbs the wave-lengths immediately above this limit, and 
their effect is very faint. A camera fitted with a quartz lens can take a 
picture in which all the wave-lengths down to 2000 A.U. produce their 
share, and hence can extend the range of the eye another octave, but the 
disadvantage of this picture is that it integrates all the different colours. 
We cannot, for example, tell whether a mark is due to light of wave-length 
3500 A.U. or 2500 A.U. Such a picture makes the ultraviolet appear the 
same as the visible would appear to a man with monochromatic vision. 
All the detail due to variety and wealth of colour is lost. Now if we could 
photograph the same objects in succession with monochromatic light of 
wave-length say 3500 A.U, 3000 A.U., 2500 A.U., and 2000 A.U, it is 
possible that a great amount of new detail might be obtained of the utmost 
value to science. It was with the purpose of obtaining monochromatic 
photographs in the ultraviolet that the present research was undertaken. 
Former investigations of this kind have been confined to a single region 
of the ultraviolet spectrum. Thin films of silver are opaque to visible and to 
the greater part of ultraviolet light but comparatively transparent to rays 
of wave-length 3160 A.U. to 3260 A.U. ; this was first observed by Foucault, 
and has been utilised by different physicists. By using a silvered quartz 
objective, R. W. Wood* was enabled to obtain photographs which were 
produced solely by these rays and which can be termed monochromatic. 
White paper still appeared white in these photographs, but powdered zinc 
oxide and Chinese white paint — whose essential constituent is zinc oxide — 
appeared black. Michaud and Tristan j* extended this investigation to a 
large number of organic and inorganic salts and to a series of flowers of all 
colours. The white inorganic salts, bismuth nitrate and cerium carbonate,, 
* Smithsonian Institution Annual Report, 1911, p. 155. 
t Arch, des Sc. p>hys. et nat, xxxiii, p. 498 (1912) ; ibid., xxxvii, p. 47 (1914). 
