884- Mr. A. Claudet 07i the principal Phcenomena of 



very far apart in the other, and sometimes they both coincide. 

 This I have tried every day during the last twelve months, and 

 I have always found the same variations. The density of the 

 atmosphere, or the colour of light, seems to have nothing to 

 do with the phenomenon, otherwise the same cause would 

 produce the same effect in both lenses. I must observe that 

 my daily experiments on my two object-glasses are made at 

 the same moment and at the same distance for each, otherwise 

 any alteration in the focal distance would disperse, more or 

 less, the photogenic rays, which is the case, as I have ascer- 

 tained. The lengthening or shortening the focus, according 

 to the distance of the object to be represented, has for effect 

 to modify the achromatism of the lenses. An optician, accord- 

 ing to M. Lerebours's calculation, can at will, in the com- 

 bination of the two glasses composing an achromatic lens, 

 adapt such curvatures or angles in both that the visual focus 

 shall coincide with the photogenic focus ; but he can obtain 

 this result only for one length of focus. The moment the 

 distance is altered, the two foci separate, because the visual 

 and photogenic rays must be refracted at different angles in 

 coming out of the lens, in order to meet at the focus given for 

 one distance of the object. If the distance is altered, the focus 

 becomes longer or shorter; and as the angle at which differ- 

 ent rays are refracted remains nearly the same, they cannot 

 meet at the new focus, and they form two images. If the 

 visual and photogenic rays were refracted parallel to each 

 other, in coming out of the lens they would always coincide^ 

 for every focus; but this is not the case. -j 



It seems, therefore, impossible that lenses can be con-,'j 

 structed in which the two foci will agree for all the various ^ 

 distances, until we have discovered two kinds of glasses, in 

 which the densities will be in the same ratio as their dispersive 

 power. There is no question so important in photography as 

 that which refers to finding the true photogenic focusof every i 

 lens for various distances. I have described the plan I haven 

 adopted for that purpose ; by means of that very simple instruri^ 

 ment, every photographer can always obtain well-defined fi 

 pictures with any object-glasses. But there is another method 

 of ascertaining the difference between the two foci, which hasj 

 been lately contrived by Mr. G. Knight of Foster Lane,)t 

 London. That gentleman has been kind enough to commu-,^ 

 nicate to me the very ingenious and simple apparatus, by, 

 which he can at once find the exact difference existing be- 

 tween the visual and photogenic focus, and place the Daguer- 

 reotype plate at the point where the photogenic focus exists. 

 1 am very glad he has entrusted me with the charge of bringing 

 his invention before the British Association. For the scientific 



