M. C. Lea — Sensitiveness of Silver Bro7mde. 



Art. LXI. — Notes on the Sensitiveness of Silver Bromide i 

 (rreen Rays as modijkd hy the Presence of other Substances 

 M. Carey Lea. 



C 



EVEEAL investigations made at different times during the 

 i years on the sensitiveness of silver bromide to rays of dif- 

 nt refrangibilities, led me to the conclusions : 1st, that its 

 sensitiveness to the different rays could be distinctly modified 

 (increased or diminished) by the presence of various bodies, 

 colored and colorless ; 2d, that no relation could be traced 

 between the color of the modifying body and the refrangi- 

 bilities of the rays to which the sensitiveness was modified. 

 During the past winter, I have carefully re-examined the ques- 

 tion, which is an important one both in its theoretical and prac- 

 tical relations, and have found my conclusions in all respects 

 confirmed. And I have during the past winter been occupied 

 with a single portion of the subject, namely, the action of the 

 green rays, as a special study. 



In this investigation I have pursued the same general method 

 as before ; that is, I have used colored glass whose transmitted 

 rays have been carefully studied with the spectroscope. There 

 is no doubt in my mind that this method of examination is 

 capable of giving results as valuable as those obtained by the 

 use of the spectrum. It may be said in fact that the subject 

 requires for its full elucidation, the use of both methods. To 

 those who may imagine that the results of the exposure to the 

 spectrum are ' the more reliable, it may be interesting to have 

 specified the weak points of that method. 



The relative strength of the impression produced by different 

 portions of the solar spectrum on a sensitive surface will always 

 depend upon the intensity of the light employed and the length 

 of exposure. It has already been proved that silver iodide 

 and bromide are sensitive to every part of the spectrum. It 

 follows that any and every part of the spectrum may be photo- 

 graphed upon plain iodide or bromide if only the exposure is 

 sufficiently prolonged. The longer the exposure, the stronger 

 will be the impression produced by the less refrangible rays as 

 compared with the more. Also, it is known that by a system 

 of masking, Prof. John Draper has succeeded in photographing 

 the whole spectrum at once ; that is to say, he was able to hold 

 back the action of the more refrangible rays until that of the 

 less was sufficiently strong. The masking was of course done 

 with red or yellow media. Now, when pigments of these 

 colors are extended over sensitive films, what is this but a sort 

 of masking, which retards the action of the more refrangible 



