318 
Transactions of the Society. 
sulphate of copper cell and intercepts the light. When the plate is in 
position and all the apparatus satisfactory, the light-intercepting card 
is removed for the required length of time and then replaced. 
Fig. 40. 
In bringing this paper to a close, I wish to acknowledge my in- 
debtedness to Mr. Edward Bausch, of the house of Bausch and 
Lomh, for practically placing the resources of the firm’s great factory 
at my disposal in the construction of the more special and fine brass- 
work so necessarily a part of the apparatus. 
