40 
Proceedings of tlie Royal Society of Edinburgh. [Sess. 
'VII. — The Absorption of Light by Inorganic Salts. No. V. : Copper 
and the Alkali Metals. By R. A. Houstoun, M.A., Ph.D., D.Sc., 
Lecturer on Physical Optics in the University of Glasgow. 
(MS. received December 1, 1911. Bead January 22, 1912.) 
The research described in this article was undertaken with the double 
purpose of testing new apparatus and of making a rapid survey of the 
absorption of light by salts of the first periodic group. 
The results described in the previous articles were obtained by three 
separate methods, viz. the thermopile in the infra-red, the spectrophoto- 
meter in the visible spectrum, and the photographic spectrophotometer in 
the ultra-violet. It would be a great simplification if the same apparatus 
could be used throughout the whole spectrum, and I therefore first of all 
devoted my attention to seeing if this were practicable. 
The ordinary spectrograph with quartz lenses and Cornu prism is 
really only suitable for one purpose, viz. the photography of the ultra- 
violet and visible spectrum, and the attainment of sharp focus is a tedious 
business. The index of refraction of quartz (ordinary ray) varies from 
1*65 to T54 within the range ordinarily used, viz. \ = T98/x to \ = '768/x. 
The collimating lens thus cannot render parallel all the rays that pass 
through it. Also, according to theory, the Cornu prism gives a double 
image, unless the light is parallel and the prism is set at minimum devia- 
tion. It is therefore very creditable that the makers get such sharp 
spectra as they do. The instrument permits, however, of no adaptation. 
It cannot readily be used with a thermopile. A glass prism ' cannot easily 
be changed for the quartz one if we are working in a region where the 
greater dispersion of glass would be acceptable. Finally, quartz does not 
transmit in the far infra-red. I have therefore devoted my attention to 
other types of instrument. 
In the fourth article of this series there is a figure showing a spectro- 
graph in which the Wadsworth mirror-prism combination is used. This 
instrument has been described elsewhere.* Its advantage is, that when 
the mirror-prism combination is rotated, the different wave-lengths come 
into focus and into minimum deviation automatically at the same point 
* Knowledge , vol. viii. p. 87 ; Studies in Light Production , The Electrician Publishing Co. 
(in preparation). 
