some Copper Salts in Aqueous Solution. 327 



centrations are, as before, given in gram equivalents in one 

 litre of solution. 



The measurements are made as follows : — The cell, either 

 empty or filled with distilled water, is adjusted before the slit 

 of the spectroscope on a suitable stand, and the ratio of the 

 intensities of the light which has passed through the upper 

 and lower halves of the trough is read, in various parts of the 

 spectrum. This gives the correction to be applied on account 

 of the apparatus- The solution is now placed in the trough, 

 and readings made through as much of the spectrum as pos- 

 sible. Ten or more single readings were made in each region 

 and the mean taken. 



The formulae which I have used in reducing the results 

 are: — (1) For the Schulz cell, 



(1-/3')*. a 

 (2) For the ordinary troughs, 



J(l-/3) 2 



0--P) 



0--p"Kl-p').a 



where 



width of lower slit ., . , . .. 



a = — nru — t ru> tbe two spectra having been made 



widtn oi upper slit l ° 



of equal brightness, and the empty trough being before 

 the slit. 

 b is the same ratio when the trough contains the solution. 

 p the fraction of the incident light absorbed by the dissolved 



substance. 

 p' = the fraction absorbed by the water. 

 /3 = loss by reflexion at surface between ends of trough 



and air. 

 /3' = loss by reflexion at surface between Schulz cube and air. 

 ft" = loss by reflexion at surface between ends of trough and 



solution. 

 $'" = loss by reflexion at surface between Schulz cube and 

 solution. 



The loss of light due to the absorption of the water was 

 calculated from the numbers of Hiifner and Albrecht*. The 

 following table contains the numbers which were used in 

 reducing the results. 



* Wied. Ann. xlii. p. 1. (1891). 



