material tried was white filter paper, s: 
t 
Chemistry and Physies. 405 
12. A Method of Measuring the Absolute Sensitiveness of 
‘iotographie Dry Plates,* by Wit1am H. Proxerine.— Within 
Some tests should be made which would determine detinitely the 
Validity of these claims, and that they should be made in such a 
manner that other persons using instruments similarly constructed 
would be able to obtain the same resu 
as dry ones, depending on the collodion, condition of the bath, 
ete., this system is very unsatisfactory. Another method em- 
ployed largely in England, ds on the use of the Warnerke 
‘ensitometer. In this instrument the light from a tablet coated 
mith tu osed t 
light, is permitted to shine through a colored transparent film of 
fraduated density upon the plate to be tested. Each degree on 
“Ga film has a number, and, after a gi 
T photographed on the plate represents the sensitiveness on an 
*mpirical scale, There are two or three objections to this instru- 
Etive for one colored light were not necessarily the most so 
or light of another color. Therefore it was evident that the sun 
~ employ the light reflected from the sky near the zenith as the 
Standard solution of silver nitrate. This was afterward replac 
* From the Proceedings of the Academy of Arts and Sciences. 
