MEMOIR XIV.] THE DAGUERREOTYPE. 211 



of this diameter answers very well for plates four inches 

 by three, reproducing the objects with the most admi- 

 rable finish, copper-plate engravings being represented in 

 the minutest particulars, and the marks of the tool be- 

 coming quite distinct under a magnifier. 



In this instance, it is true, owing to the magnitude of 

 the focal length compared with the aperture, but little 

 difficulty ensues from chromatic aberration ; but when 

 with the same focal length the aperture is increased to 

 three or four inches, the dispersion becomes very sen- 

 sible, and yet good proofs can be procured by working 

 in the method here indicated, the chief difficulty then 

 arising from spherical aberration. 



It has already been stated that the ray of maximum 

 action for the daguerreotype, when colorless French 

 plate-glass is used, lies probably within the indigo space: 

 it therefore follows that the length of the camera should 

 be diminished, after arranging it to the luminous focus. 

 The importance of this has been pointed out in a paper 

 by Mr. Towson ; I was, however, in the habit of using 

 that adjustment before reading the suggestions contain- 

 ed in his communication. The amount of shortening to 

 be given to the camera, where the lens is fifteen inches 

 focus, does not commonly exceed three tenths of an inch. 

 If the luminous focus be used, the proof comes out indis- 

 tinctly. 



In the subsequent process of mercurializing, it is of lit- 

 tle importance what the angular position may be. Sev- 

 eral experimenters were for a time under the idea that 

 an angle of 45 or 48 is a necessary inclination, in order 

 that the plate should take the vapor; this arose from a 

 misinterpretation of the printed account. Plates mercu- 

 rialize equally well in a horizontal as in any other posi- 

 tion ; perhaps a slight inclination may be of advantage, 

 in allowing the vapor to flow with uniformity over the 



