180 Proceedings of the Royal Society of Edinburgh. [Sess. 
increased, for the eye of the observer need never leave the telescope of the 
polarimeter, nor is he required to remove his hand from the screw actuating 
the Nicol dial. 
I am accustomed to use a “ backed ordinary ” plate, and a hydroquinone 
developer, but any of the usual plates and developers may be employed. 
Fig. 4. 
No special care need be taken with the development of the plate, because 
all that it is necessary to obtain is a negative which is legible, and this can 
be secured without any difficulty. “ Prints ” of the negatives need not be 
made, as it is so easy to store the latter. The strip of blank space which 
appears at the foot of each negative is convenient for the purpose of writing 
particulars upon relating to the experiment whose readings it records. 
