on Silver Iodide and Bromide. 74 
tive serves in each case as a measure of the degree of sensitive- 
ness. This has a particular importance when a development 
process is used, because even with the utmost care, there will 
sometimes be a discoloration arising from the action of the 
sion, exactly where it is most able to induce error. The em- 
ployment of a negative avoids this difficulty, and if its image 
has a great variety of tones, the observer is able to measure 
very closely the relative extent to which it is reproduced on 
the sensitive surface, 
development by means of gallic acid and silver nitrate, con- 
trolled by acetic acid. 
8 
confirmed the first conclusions as to the absolute purity of the 
Specimen used. 
