104 SUMMARY OF CURRENT RESEARCHES RELATING TO 



a prism, and will do the same, but to a different extent, to the wave 

 polarised in the perpendicular direction. If the beam of light be 

 analysed by means of a grating, the spectrum lines all appear doubled, 

 the two components being oppositely polarised. The shift, so produced, 

 can be measured, and i therefore obtained ; hence, C\ is known. 

 Similarly C 2 can be found. Thus the absolute changes in the two 

 indices of refraction can be calculated, and this not only for one kind of 

 light, but for as many kinds at once as there are lines visible in the 

 spectrum under observation. 



Atlas of Absorption Spectra. — This is a very excellent collection, 

 by H. S. Uhler and R. W. Wood, of photographs of absorption spectra. 

 For their production a mirror and a concave grating were employed, the 

 light from the source passing through a wedge-shaped layer of the 

 solution under investigation, after reflection from the mirror. This 

 layer is placed horizontally over the slit, which is also horizontal, the 

 path of the rays being vertical. Through a tilting arrangement adapted 

 to the containing cell its angle is variable. Its edge is at right angles 

 and in the same plane as the direction of the slit. 



Three exposures of different but relatively uniform duration were 

 usually given to each plate. As source of light a Nernst lamp was used 

 for wave-lengths between 0'65/x and 0*326^, and for wave-lengths 

 between - 326/x and 0*2 //., and as an index a specially arranged spark 

 discharge between electrodes of an alloy of cadmium and zinc on the 

 one hand, and of brass on the other was used, the spark spectrum photo- 

 graph being superposed on that from the Nernst burner. 



The authors recommend water as a solvent of the substances investi- 

 gated as being free from absorption in the ultra-violet. But a recent 

 determination of the refractive indices of water has shown that for the 

 extreme wave-length 0'185/x. this is not the case.* 



As Professor Wood points out in the introduction, several workers 

 have made a series of photographs of absorption spectra previously, but 

 with them, the end in view was not a book of reference. Work of this 

 kind was undertaken under the auspices of the Royal Microscopical 

 Society in 1893, the outcome of which were the F and 6 line screens 

 described subsequently in this Journal^ and also a screen for use in 

 orthochromatic photography 4 On that occasion the sun alone was used 

 as light source, the fine absorption lines of the solar spectrum in no way 

 interfering with the observation of the comparatively broad absorption 

 bands of the substances under investigation, and showing their position 

 at a glance. In this way most of the anilines now described, besides 

 others, and the principal salts of copper and chromium were then photo- 

 graphed. But the present authors, by employing light from the artificial 

 sources described, have extended the range to the ultra-violet, and finally 

 have published their work, together with a descriptive table of the 

 substances investigated, and of the results obtained. This table gives 

 the commercial as well as the chemical name of each, and also that of 



* 



Proc. Roy. Soc, 1906. 

 t See this Journal, 1894, pp. 164-7, and 1S95, pp. 145-7. 

 % Journ. Roy. Photo. Soc, 1895. 



