On a Photographic Process. 29 



it passes tlirough a small air space in the mercury of the 

 thermometers, and by means of lenses a bright image of this 

 space is focnssed on to the cylinder. The photographic results 

 obtained are simple, consisting of straight or curved lines, or 

 a regularly blackened surface without toning or shading of 

 any kind, as will be seen by specimens on the table. 



It is, however, merely to the methods of preparing the 

 sensitive paper that I wish to direct your attention to-night. 

 At first the method we adopted was that known as Crooke's 

 modification of the old wax-paper process of Gray ; a modifi- 

 cation arrived at after considerable experiment to ascertain 

 what particular salts of silver, and their proportion, would be 

 most sensitive to ordinary artificial lights. The method is 

 fully described in the British Association report of 1859, and 

 was that subsequently adopted at the observatories at Kew 

 and Oxford. In this process the paper is first soaked in 

 melted wax, and superfluous wax afterwards removed by hot 

 pressing. By this means it was intended to give the paper a 

 closer and smoother surface, and also to render it transparent, 

 so that copies of any pictures or impressions on it might be 

 photographically obtained. 



The paper thus prepared and cut to the proper size is 

 immersed in a bath composed of proper proportions of Iodide 

 and Bromide of Potassium, with enough free Iodine added 

 to render the solution of a port wine colour. After immer- 

 sion for several hours in this bath the paper becomes of a dark 

 reddish brown, and can be kept in this state for almost any 

 length of time. The next part of the process is to render the 

 paper sensitive to the action of light, which is accomplished 

 by floating them in a bath of Acetic Nitrate of Silver on 

 which, after a few minutes, they become of the delicate straw 

 colour of Iodide and Bromide of Silver; in this state they 

 can be preserved in the dark for some days without much 

 deterioration, but in practice they seldom keep so much 

 as a week before using. It is in this condition they are 

 placed upon the cylinders for registering. 



When removed from the cylinder, after the 24 or 48 hours' 

 exposure to the dots or lines of light, the paper appears 

 just the same as when placed upon them, no impression 

 is visible till after the next process, developTnent. This is 

 done by floating them upon a solution of Gallic Acid with 

 Nitrate of Silver and Acetic Acid [a film of which is poured on 

 an accurately levelled sheet of plate-glass], for a period of time 

 varying from two to three hours, depending on the tempera- 



