164 On the Opacity of the Developed Photographic Image. 



the ground glass. The position of the slit in regard to the scale 

 was noted, and the intensity of light transmitted calculated by 

 the ordinary formula. Each strip was compared six times — 

 three times by myself, and three times by an assistant. A mean 

 of the six readings was taken as correct. 



Fig. 2. 



Era* 



A and B are the curves given by the images on the dry plates. 



C and D are the curves given by the image on wet plates. 



The dotted lines indicate the line whose ordinates give an arithmetic 

 progression of transparency, E F being unity or transparency. 



F G represents the length of the strips examined, and therefore the 

 varying intensity of light, F being zero and G the maximum. 



Regarding the curves given by the dry plates, if we sup- 

 pose that varying intensities of light cause a corresponding re- 

 duction of the bromide of silver after development, it can be 

 easily demonstrated that the intensity of light passing through 

 the image after clearing away the unaltered bromide would be 



V = n.€~ k \ 



W 



where n and k are constants depending on the thickness and 

 opacity of the bromide film, and I the intensity of the light pro- 

 ducing any one part of the image — that is, on the supposition 

 that the image is formed of matter continuous but of varying 

 density. This is not the case, but there is an approximation to it. 

 Under the same supposition we can assume that there is a function 

 of time into a function of intensity of light acting on an infi- 

 nitely thin layer of the bromide of silver which will cause an 

 entire reduction of the bromide on development : this we might 

 call a state of saturation. In the image of the star there may 

 be some point where the upper layer of bromide (of infinite 

 thinness) is saturated. From that point along the image to be 

 produced by the higher intensities the whole surface is satu- 

 rated, and the saturation must gradually approach the bottom 

 surface. From the point where the whole depth of the layer is 

 saturated, along the image to be produced by still higher inten- 



