S84f Mr. Towson on the proper Focus for the Daguerreotype, 



the importance of obtaining the chemical focus of a lens, I 

 have inclosed two street viev/s taken on the " improved photo- 

 graphic paper" sold by Mr. Richards of this town. This 

 preparation produces lights which correspond with lights 

 and shades with shades, consequently the effect of a correct 

 focus is more perceptible than would be the case on papers 

 that reverse the tints. The subject of both views is the same; 

 the paper of each is from the same piece ; and the times and 

 the lights employed in taking them were as similar as possible, 

 the difference of effect being solely produced by No. 2 having 

 been placed in the mean luminous focus, but No. 1 in the 

 mean chemical focus, discovered by the above formula. On 

 observing the very great difference between the two views, 

 the question immediately occurs, how then does Daguerre 

 produce such clear pictures if he uses the wrong "focus? 

 When however we observe the imperfect view, No. 2, we are 

 not to conclude that the luminous focus always produces so 

 little distinctness. During the summer months I have, together 

 with Mr. Hunt of this town, devoted considerable attention 

 to the practice of the photographic art, and have succeeded 

 in obtaining many very tolerably distinct views, although we 

 used the luminous focus of the lens. This we effected by re- 

 ducing the diameter of the lens or stop to a considerable ex- 

 tent, but by so doing we delayed the process of taking the 

 view. This is also the mode by which Daguerre in a great 

 measure neutralizes the effect of the imperfect focus which 

 it appears he is in the habit of using. By thus reducing the 

 size of the lens of a camera, that aberration'of the glass which 

 arises from the use of a wrong focus is diminished in direct 

 proportion to the squares of the diameter of the lens or stop, 

 but the number of rays transmitted is thereby reduced in the 

 same proportion. The diameter of the stop of the camera 

 employed in drawing the inclosed views was equal to one- 

 sixth of the focal length of the lens, whereas it appears from 

 the description of Daguerre's camera that his lens is of less 

 diameter than yj^th of its focal length, and the engraving 

 which represents his camera shows a stop of one half that 

 diameter. This being the case the aberration arising from 

 the incorrect focus is reduced to ^^th of the amount shown in 

 view, No. 2. But by thus reducing the number of rays trans- 

 mitted, much of the advantage which would arise from the 



servation might be exemplified by reference to several interesting facts, but 

 in so doing we should prematurely anticipate some of the results of an 

 investigation, which my friend, Mr. R. Hunt, is now making relative to the 

 power which various transparent media possess of transmitting chemical 

 rays. 



