25 



substitution of gelatine for collodion, (though of course with 

 correspondingalterations in the proportions used in formulae), 

 a marvellous change has been worked in photography. 

 Where with collodion dry plates, (especially those which had 

 been treated with beer as a preservative), hours were neces- 

 sary to obtain a picture of a landscape, less than as many 

 seconds, and even a fraction of a second, now suffice; and it 

 is not only the time of exposure that is, I may almost say, 

 annihilated. The other conditions to obtaining sun pictures, 

 such as for some subjects the necessity for bright sunshine, 

 and the impossibility of photographing interiors lighted 

 through stained glass, as well as the vexatious blemishes 

 which often spoiled an otherwise satisfactory negative, have 

 disappeared. 



A short description of the preparation of gelatine dry 

 plates will bear out my statement that all this depends on no 

 fundamental change in dry plate formulae. In working with 

 gelatine, nitrate of silver is dissolved as before, (though this 

 time in water), and mixed very gradually, and with constant 

 stirring, with a soluble bromide which has been dissolved with 

 a small quantity of gelatine in hot water. This is found to 

 produce silver bromide in an excessively fine state of division, 

 though it is not yet very sensitive to light. The mixture is 

 however now generally boiled, or sometimes treated with 

 ammonia, and it has been allowed to remain for some time in 

 a cold state. All these treatments result in the silver bromide 

 becoming highly sensitive, though I must confess that the 

 reason for this is not clear to me. No chemical change ap- 

 pears to take place in it, though it alters considerably in 

 color, and the ultimate tint which it attains, a kind of blue 

 grey, is not the most sensitive. However this may be, when 

 the emulsion is sufficiently ripe, as it is termed, it is either 

 allowed to solidify by cooling and washed in a dry state, like 

 collodion emulsion, or the silver bromide is precipitated and 

 re-emulsified with fresh gelatine and water. After washing, it 

 is ready for use, with the addition of some more gelatine to 

 enable it to set properly, and it can be kept for an indefinite 



