12 LankSHEAR, Quantitative Absorption Spectra. 



i.e., points where log. ///^^the log. of the aperture ratio. 

 For simplicity, the adjustable sector is graduated logar- 

 ithmically. From a knowledge of d, the length of solution, 

 and c, the concentration, and log. ///j e, the extinction 

 coefficient, is readily calculated from the formulas 

 fi = I . 10 "'"'^ where e is the extinction coefficient, c the 

 concentration, and d the thickness. 



The chief assumption in this method is the truth of 

 Schwartzschild's formula. This has been obviated by 

 calibrating each batch of plates used, and often various 

 plates of the same batch, with a piece of glass of known 

 extinction coefficient in the ultraviolet. Usually a small 

 correction has to be applied to the value of log. ///^ = log. 

 of aperture ratio. Variations in the intensity of the light 

 source have no effect since the standard and transmitted 

 beam are used simultaneously. Some disadvantages of 

 the method are the necessity of frequently calibrating the 

 plates, the long exposures required for the extreme ultra- 

 violet (of the order of 500 sees.) and the short length of 

 solution which can be used, 5 cms. as a maximum. The 

 advantages are : independence of variation of the light 

 source, ease of reading, for the plates give the wave 

 length and log. ///, directly, and the automatic allowance 

 for the effect of the solvent. 



It is hoped shortl)' to describe an instrument in which 

 the Baly tube can be used, which gives a steady instead 

 of an intermittent beam on the slit, thus obviating the 

 Schwartzschild correction, and, finally, focussing the image 

 of the light source on the slit instead of the object glass. 



Chemical Department, 



The University, Manchicstkr. 



