MODIFICATIONS IN THE DAGUERREOTYPE. 



do not evolve correspondingly, nor can they be well developed, except at the risk of 

 solarizing the picture. Now a plate that has been kept for several hours is by no 

 means so liable to these effects : I do not pretend to give any reason for this, but 

 merely mention it as a fact of considerable importance to the travelling Daguerreotyper; 

 he will find that the iodine does not lose its sensitiveness in many days. 



535. In a paper read before the Royal Society, of which an abstract is given in the 

 April number of this Journal for the present year (p. 333), Sir JOHN HERSCHEL states 

 that there is an absolute necessity of a perfect achromaticity in the object-glass of a 

 photographic camera. M. DAGUERRE appears to have been under the same impression, 

 and recommends in his published account such an object-glass. 



536. All the rays of light, with, perhaps, the exception of the yellow, leave an im- 

 pression 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 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. 

 "\ ision 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, 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 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 withdrawn at a certain period, when the rays that have a maximum energy have just 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. 



537. 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 

 witb 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 ; copperplate engravings 

 being represented in the minutest particulars, and the marks of the tool becoming quite 

 distinct under the magnifier. 



538. 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 disper- 

 sion becomes very sensible, and yet good proofs can be procured by working in the 

 method here indicated, the chief difficulty then arising from spherical aberration. 



539. It has already been stated that the ray of maximum action for the Daguerreo- 

 type, when colourless 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 is pointed out in a paper by Mr. 

 TOWSON, inserted in this Journal last year ; I was, however, in the habit of using this 



