M. C. Lea — Sensitiveness of Silver Bromide. 461 



equal time, the development must be made by a bath applied 

 to both portions equally and for an equal time. If these condi- 

 tions are not maintained, the result will be deceptive. The 

 intensity o^Wght must be the same and the duration of exposure, 

 because the relative efifect of the dift'erent rays will always be 

 proportionate to them. The same plate exposed to the same 

 image for a double time, or for an equal time to an image of 

 double intensity, will give a final result in which the relative 

 strengths are totally difi'erent. The relative strength of different 

 parts of the image 'is also largely modified by the development. 

 I am therefore justified in affirming that no strictly comparative 

 trial can be made except the two images, on the colored and uu- 

 colored films, be received on the same plate, simultaneously 

 and for the same time, and be simultaneously and equally de- 

 veloped. 



These conditions have never yet been fulfilled in the case of 

 spectral observations. By the use of colored glass they may be 

 maintained with absolute exactness. And when colored media 

 can be obtained which exclude all but a given band of consecu- 

 tive rays, the effect of colored substances added to the film of 

 silver bromide, in modifying its sensitiveness to this band of 

 rays, may be accurately fixed. Having previously, with the 

 aid of the spectroscope, determined the exact character of this 

 band, we are enabled to speak very decisively of the action of 

 the rays of which it is composed. 



In the present investigation I have limited myself to a single 

 question : Does there exist any red substance which is capable 

 of increasing the sensitiveness of silver bromide to the green 

 rays ? 



For the purpose of this examination, I used three thicknesses 

 of very dark green glass, the limiting wave-lengths of which 

 had been many times measured with closely corresponding re- 

 sults ; the widest variation was two minutes of arc, which when 

 it is considered how gradually the band fades out at its borders, 

 and how very faint the illumination is at its extreme limits, is 

 fully as close an approximation as could be expected. The ex- 

 treme limits of the band, measured to the limit of visibility, 

 were A497 and A58L But the extremities showed an illumina- 

 tion too feeble to have any effectual result Cutting off the 

 very faint light, the band was reduced to A517 on one side, and 

 ^569 on the other, and the raj^s between these limits may be 

 taken as those whose results were observed. 



In order to register the effects produced, a glass negative of 

 a suitable character was placed under these three glasses, and 

 under the negative, a glass carrying the film to parts of which 

 Am. Jour. Sci.— Third Series, Vol. XI, No. 66,— Jusb, 1876. 



