210 THE PROCESS OF DAGUERREOTYPE. [MEMOIR XIV. 



states that there is an absolute necessity of a perfect 

 achromaticity in the object-glass of a photographic cam- 

 era. M. Daguerre appears to have been under the same 

 impression, and recommends in his published account 

 such an object-glass. 



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

 yellow, leave an impression on the iodide of silver. The 

 less refrangible rays, however, act much more slowly 

 than those at the opposite end of the spectrum. In the 

 common 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 there- 

 fore performed independently of time, the eye catching 

 all the colors of the spectrum with equal facility and 

 with equal speed. But it is not so with these photo- 

 genic preparations. In the action of light upon them, 

 time enters as an element ; the blue ray may have effect- 

 ed its full change, while the red is yet only beginning 

 slowly to act ; and the red may have completed its 

 change before the yellow has made any sensible impres- 

 sion. 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 of fine photographs ; for if the plate be withdrawn at a 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 tem- 

 porary monochromatic light. 



Upon these principles I constructed the camera I am 

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

 matic lens. Some of the finest proofs were procured 

 with a common spectacle lens, of fourteen inches focus, 

 arranged at the end of a ci^ar-box as a camera; a lens 



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