Phologra'phy in the Daguerreotype Process. 383 



between the angles inscribed in the curves either of the crown- 

 or flint-glass, he could render at will the photogenic focus 

 longer or shorter than the visual focus, and by the same 

 means could bring them to the same point. There is no ques- 

 tion that M. Lerebours was right as far as the result referred 

 to the chromatic correction ; but if, according to the density 

 of the two glassesjcertain curvatures are required to correct the 

 spherical aberrations, these curvatures cannot be altered with 

 impunity only for the purpose of changing the directions of 

 the most refrangible rays. For this reason 1 have always 

 preferred lenses in which the spherical aberration is the most 

 perfectly corrected, without caring whether the photogenic 

 rays coincided or not with the visual rays, having the means of 

 ascertaining how I could obtain on my Daguerreotype plate 

 the best-defined image. In fact, from my own observation 

 that the red, orange, and yellow rays are antagonistic to the 

 photogenic rays, and that the last rays have a greater power 

 when the former are proportionately less abundant, I am of 

 opinion that when the photogenic rays are only condensed on 

 the plate, and the others are dispersed on the space more or 

 less distant from the photogenic point, the action is more 

 rapid. Rapidity being the principal object in photography, 

 1 prefer lenses in which the two foci are separated, although 

 the operation is a little more difficult, and requires consider- 

 able care. 



The question of the photogenic focus is involved in another 

 kind of mystery, which requires some attention. I have found 

 that with the same lenses there exists a constant variation in 

 the distance between the two foci. They are never in the 

 same relation to each other : they are sometimes more or less 

 separate; in some lights they are very distant, and in some 

 others they are very near and even coincide. For this reason 

 1 constantly try their position before I operate. I have not 

 been able to discover the cause of that singular phsenomenon, 

 but I can state positively that it exists. At first I thought 

 that variations in the density of the atmosphere might produce 

 the alteration in the distance between the two foci ; or that 

 when the yellow rays were more or less abundant, the visual 

 rays were refracted on different points on the axis of the foci, 

 according to the mean refrangibility of the rays composing 

 white light at the moment. But a new experiment has proved 

 to me that these could not be the real causes of the variation. 

 I generally employ two object-glasses ; one of shorter focus 

 for small pictures, and the other of longer focus for larger 

 images. In both the photogenic focus is longer than the visual 

 focus; but when they are much separated in one they are less 

 so in the other : sometimes when they coincide in one, they »re 



