158 ANNUAL OF SCIENTIFIC DISCOVERY, 



s* 



operandi is this : having first filled the troughs with their respective 

 liquids, the camera is placed upon its table or support, and this again 

 upon a stand which is usually required to give it a due elevation from 

 the ground. The camera is pointed at the object, and a sheet of 

 ground glass is placed in the frame from behind, to obtain the focus, 

 and is then removed, and a sheet of prepared iodized paper, or a 

 plate of iodized glass (which of course must not be at all sensitive) is 

 put in its place. A door is then shut at the back of the camera, 

 which places the prepared paper of glass in the dark. The camera is 

 then moved on its support to the mark indicating the trough of nitrate 

 of silver. The object-glass of the camera is then closed. The ope- 

 rator then takes hold of the frame by its handle, and pushes it down 

 into the trough below, which he is enabled to do by reason of a narrow 

 slit in the bottom of the camera, which allows a passage. He then 

 draws it up again immediately. He then opens the object-glass of the 

 camera, and after a due time closes it again. He then moves the 

 camera on its support to the mark indicating gallic acid, or sulphate 

 of iron. He then, as before, pushes the frame down and lifts it up 

 again, either immediately or after a due length of time. He then, in 

 a similar way, drops the frame into and out of the trough of water. 

 He then opens the door at the back of the camera, and takes out and 

 examines the picture he has obtained, which for that purpose he may 

 freely expose to the light. If not satisfied with it, he tries again, cor- 

 recting his process by his first experience. But if he is satisfied with 

 his picture, he deposits it in his box. It is not yet quite finished, but 

 the finishing process is deferred without inconvenience until after his 

 return in the evening. In practice, I find that this simple arrange- 

 ment works delightfully, and I should be glad to be allowed to name 

 it the Traveller's Camera." 



ON THE SOLAR AGENCY PRODUCING CHEMICAL CHANGE. 



FROM Mr. Hunt's late work on Photography, we copy the following- 

 passage relative to the solar agency, which produces the chemical 

 change. 



" The operation of the antagonistic forces of light is somewhat 

 remarkably shown over different regions of the earth. Advancing 

 from our own lands towards the tropics, it is found that the difficulties 

 of obtaining pictures by the solar influences increase ; and, under the 

 action of the glowing light of equatorial climes, a much longer period 

 is required for impressing a photograph, than is occupied in the pro- 

 cess either in London or Paris. It has been stated by Dr. Draper, 

 that in his progress from New York to the Southern States, he found 

 the space protected from chemical change by the yellow rays regularly 

 increasing. The same result is apparent in the differences between 

 the spring and summer. Usually in April and March photographs 

 are more readily obtained, than in June and July. It is worthy of 

 notice, that the morning sun, between the hours of eight and twelve, 

 produces much better effects than can be obtained after the hour of 





