M. C. Lea — Sensitiveness of Silver Bromide. 



rays while freely permitting that of the less ? Even supposing 

 ■ pnents to be chemically inert they would 

 the blue end of the spectrum and render : 

 jer exposure to obtain both togeth( " 



exposure would not be absolutely essential, 



these pigments to be chemically inert they would check the 

 action of the blue end of the spectrum and render it possible 

 . longer exposure to obtain both together. Even a longer 

 Qd not be r ' ' 



^ iopmentma^ ^ , ^ ^ 



by the diminished impression of the blue end. 



If, then, it is alleged that by coloring a film of silver bromide 

 with red pigments, the sensitiveness to yellow rays is increased. 



moved to reply that such a result is no proof of 

 a chemical or photochemical action exerted by the red pigment ; 

 that precisely the same result might be expected if the red sub- 

 stance were chemically inert, or if it were extended over a glass 

 surface and simply interposed in the path of the rays, between 

 the prism and the film, without even coming into contact with 

 the latter. Many results that have been published are liable to 

 this fatal objection. It has been proposed to modify the form 

 of experiment by applying the color to the front of the plate 

 and exposing on the back, through the glass. But even this 

 does not remove the difficulty. The collodion film containing 

 the silver bromide is exceedingly thin and when moistened, 

 very porous and absorbent. Any soluble color applied, dissolved 

 in water or alcohol, instantly penetrates it through and through 

 and even moistens the glass under it. And all the colors 

 mentioned by other experimenters as having been subjected to 

 this experiment, are soluble. It is on these results that theories 

 have been based, and they are all susceptible of full and com- 

 plete explanation in the manner just mentioned. 



In matters of photographic experiment such as these now 

 under discussion, there are alwavs three distinct factors : The 

 sensitiveness of the matter, the force of the impression (depend- 

 ing upon the intensity of the light and the length of exposure), 

 and the development. This ] " ' ' ' ^ ^-^ *■ 



makes the investigation difficult 



icHuiLs uy varying xue ueveiopiueuL, or witii au lucunv."- — - 

 and development, by varying the exposure. The amount of 

 error and deception liable to be introduced in this way is known 

 to none but those made familiar with it by expenence, and 

 consequently for accurate results, these sources of error must be 

 eliminated. There is but one way of doing this : a film must 

 be taken, one portion of it must be washed over with the sub- 

 stance whose action is to be studied, and then the two portions, 

 the plain and that which has been treated, must be sim- 

 ultaneously exposed and simultaneously developed. Both por 

 tions must receive the same ravs, of equal intensity and t.-r :.u 



