220 Professor Draper on the Process of Daguerreotype, 



All the rays of light, with perhaps the exception of the yel- 

 low, leave an impression on the iodide of silver. The less 

 refrangible rays, however, act much more slowly than those 

 which are at the opposite end of the spectrum. In the com- 

 mon kinds of glass, the most energetic action takes place in 

 the indigo, or on the boundary of the blue. Now the retina 

 receives an impression with equal facility from each of the 

 different rays, the yellow light acting as quickly upon it as the 

 red or the blue. Vision is therefore performed independently 

 of time, the eye catching all the colours of the spectrum with 

 equal facility and with equal speed. But it is not so with these 

 photogenic preparations. In the action of light upon them, 

 time enters as an element ; the blue ray may have effected its 

 full change, whilst the red is yet only beginning slowly to act ; 

 and the red may have completed its change before the yellow 

 has made any sensible impression. On these principles, it is 

 plain that an achromatic object-glass is by no means essential 

 for the production of fine photographs ; for if the plate be 

 withdrawn at a certain period, when the rays that have a 

 maximum energy have just completed their action, those that 

 are more dispersed but of slower effect, will not have had time 

 to leave any stain. We work, in fact, with a temporary 

 monochromatic light. 



Upon these principles I constructed the camera which I 

 am in the habit of using, with a double convex non-achromatic 

 lens. Some of the finest proofs were procured with a common 

 spectacle lens, of fourteen inches focus, arranged at the end of 

 a cigar-box as a camera ; a lens of this diameter answers very 

 well for plates four inches by three, reproducing the objects 

 with the most admirable finish, copper-plate engravings being 

 represented in the minutest particulars, and the marks of the 

 tool becoming quite distinct under the 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, 

 then the dispersion becomes very sensible, and yet good proofs 

 can be procured, by working in the method here indicated, 

 the chief difficulty then arising from spherical aberra- 

 tion. 



It has already been stated, that the ray of maximum action 

 for the Daguerreotype, when colourless French plate-glass is 

 used, lies probably within the indigo space: it therefore fol- 

 lows, that the length of the camera should be diminished, 

 after arranging it to the luminous focus. The importance of 

 this is pointed out in a paper by Mr. Tow son, inserted in this 

 t Journal last year ; I was, however, in the habit of using this ad- 





