Manchester Memoirs, Vol. Iviii. (19 14), No. 15. ii 



Messrs. Hilgers, a beam of light is divided into two equal 

 portions by a pair of deflecting" windows of quartz. The 

 lower beam then passes through a rotating sector of fixed 

 aperture, and through a cell of definite length, containing 

 the solution under examination. It then falls on a 

 biprism fixed to the slit of the spectrograph. The upper 

 beam after deflection passes through a rotating sector, 

 the aperture of which is adjustable. It then traverses a 

 cell identical with the lower one containing the solvent 

 and falls on the biprism. The two images come to a 



Fig. 2. — General appearance of apparatus. 



focus on the object glass of the spectrograph, and form 

 on the plate two spectra side by side and just touching. 

 A blank photograph gives two spectra of equal intensity 

 over their whole range. By means of the adjustable 

 sector the strength of the standard beam can be cut 

 down by any known amount, the logarithm of the ratio 

 of the two apertures giving log. ///j. As in the Henri 

 method, those points are read ofl" at which the standard 

 beam and the transmitted beam are of equal intensity, 



