A Manual Of Photography

ON THE THEORY OF THE DAGUERREOTYPE. 151 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 com- posing white light at the moment. But a new experiment lias 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 actinic 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 are 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 witli 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 actinic rays, which is the case, as it is easy to prove. 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, according to M. Lere- bours’ calculation, can at will, in the combination of the two glasses composing an achromatic lens, adapt sueli curvatures or angles in both that the visual focus shall coincide with the actinic focus; but he can obtain this result only lor one length of focus. The moment the distance is altered, the two foci separate, because the visual and actinic rays must be re- fracted 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 different rays are refracted remains nearly the same, they cannot meet at the new focus, and they form two images. If the visual and actinic rays were re- fracted parallel to each other, in coming out of the lens they would always coincide for every focus ; but this is not the case. It seems therefore, impossible that lenses can be constructed in which the two foci will agree for all the various distances, until we have discovered two kinds of glasses in which the densities or the refractive power will be in the same ratio as the dispersive ^The experiments of Mr. Stokes, to whieli reference has already been made, appear to prove a set of conditions, new to our