[ 379 ] 



XLII. Note on the Selective Absorption of Light by Optical 

 Gla*s and Calc-spar. By Edward L. Nichols and 

 Benjamin W. Snow*. 



IN the course of some recent experiments f, the authors 

 had occasion to measure the absorption which the rays 

 from an incandescent lamp suffered in passing through a lens 

 and a pair of Nicol's prisms. A number of observations 

 upon the absorption of light by lenses have already been 

 published, but they deal for the most part with the total 

 absorption. Vierordt J, however, aud very recently Kruess §, 

 have measured the amount of absorption that takes place in 

 each regioxi of the visible spectrum, and Abney and Festing || 

 have shown that such absorption may introduce very im- 

 portant errors into colour photometry. 



For the details of the method by means of which the 

 selective absorption in glass and calcite was determined, we 

 must refer the reader to the article already cited. The lens 

 and prism in question were mounted before the slit of a 

 spectrophotometer under conditions which made it possible 

 to compare the light from a glow lamp with rays from the 

 same lamp after passage through the lens, or through both 

 lens and prisms. In every other respect the two sets of rays 

 under comparison were subjected to precisely the same 

 treatment, in their path from the lamp to the eye of the 

 observer. Any differences in the character of their spectra 

 were ascribable, therefore, to losses incurred in traversing 

 the lens, on the one hand, or to the combined action of the 

 glass and the calcite on the other. No attempt was made to 

 distinguish between losses due to absorption and those 

 resulting from reflexion at the various surfaces. 



In presenting our results, light of the region of the D line 

 (\ = 5890) is taken as a standard of reference. The amount 

 of light of this wave-length, transmitted by the lens, or by 

 the Nicol's prisms, respectively, is taken as unity, and the 

 relative proportions of light of other wave-lengths trans- 

 mitted are given in terms of that quantity. This method 

 expresses the character of the change. The results are 



* Communicated by the Authors. 

 t Phil. Mag., vol. xxxii. p. 406. 

 X Die quantitative Spectralanalyse, 1876, p. 113. 

 § Gerhard und Hugo Kreuss ; Kolorimetrie, 1891, p. 252. 

 || " Colour Photometry," 1888, Philosophical Transactions, vol. clxxix. 

 p. 549. 



