1 882. 177 Trans. N. Y. Ac. ScL 



the object would be produced. But such a photograph would fade if 

 brought out into the light ; besides, it would not be very intense. It is 

 necessary, therefore, to devise some method by which the change pro- 

 duced by the light can be strengthened, and then fixed. In practice, 

 the sensitive compound is obtained upon a glass plate in one of two 

 ways ; either by coating the plate with a tilm of some substance that 

 is permeable to fluids, such as collodion containing a bromide or an 

 iodide, and then dipping the plate thus coated into a bath of silver ni- 

 trate, whereby the silver bromide or iodide is formed within the pores 

 of the collodion ; or else by precipitating the silver compound in a so- 

 lution of collodion, gelatin or other medium, in such a manner that the 

 particles will be held in suspension in a state ot minute division. By 

 the latter process a so-called emulsion is formed, which is poured over 

 a plate and adowed to dry before use. The causes which affect the 

 sensitiveness of the bromide are still very obscure. It is probable that 

 this is controlled by the physical conditions under which it is formed ; 

 in other words, the size of the particles of bromide in the emulsion. 

 The smaller the particles, the more sensitive they are. Yet it seems 

 possible to increase the sensitiveness of an emulsion, by causing the 

 minute particles to become mechanically aggregated into larger masses. 

 At least, this is the explanation suggested by the able experimentalist, 

 Captain Abney, to account tor the greater sensitiveness of a gelatin 

 emulsion after boiling. This aggregation of the particles cannot in 

 any wise affect'the sensitiveness of the constituent molecules ; but, if we 

 can understand how it causes the film to be more sensitive, we will gam 

 an insight into a molecular change which may be said to lie as the 

 foundation of the process of strengthening the image, which is the pro- 

 cess commonly known as development. 



It has been stated that a perfectly uniform layer of the sensitive silver 

 salt upon a plate would show an image after exposure in the camera. 

 This image results from the decomposition of the sensi ive salt into a 

 sub-bromide, sub-iodide or sub-chloride, as the case may be. the bromine, 

 iodine or chlorine being set free ; thus 2 (Ag Br)= AgoBr + Br. 

 The change of color, resulting from a short exposure in the camera, is 

 not great ; but it is possible to so prepare a plate that the chemical 

 change produced by light in a fractional part of a second sufhcr-s to 

 produce an image. Under such circumstances the chemical change 

 must be very superficial, and the de:omposition too slight for the eye 

 to detect. Hence, after a plate is properly exposed in the camera, no 

 image is visible upon it. To make the image visible, it must be 

 strengthened or developed. 



Before describing the process of development, I will illustrate ttie 

 process of making an emulsion of silver bromide for the drv-plate pro- 



