13 de Junho
ABSTRACTS OF THE PAPERS,
ROYAL SOCIETY OF LONDON
Proceedings of the royal society
3. “ On the Non-coincidence of the Focus of the Photogenic Rays with that of the Visual Rays of the Solar Spectrum.” By M. A. Claudet. Communicated by S. Hunter Christie, Esq., sec. R. S., &c.
After detailing the difficulties he had met with in obtaining perfect pictures when a lens, accurately corrected for spherical and chromatic aberration, was employed in the Daguerreotype process, the author states that in order to obtain a clear and well-defined image of any object on the Daguerreotype plate, he generally found it necessary to adjust the focus on the ground glass by another object brought considerably nearer to the camera than the object whose picture was required. When this adjustment is made, he proceeds to apply the principle practically to the taking of portraits. He finds that in achromatic object-glasses the focus of photogenic action is not coincident with the visual focus; and the distance between these two foci varies according to the nature of the combination of the glasses, to their different dispersive powers, and to the degree of intensity of the light. By attentation to these circumstances in accurately adjusting the Daguerreotype plate to the situation of the focus of the photogenic rays, the author has succeeded in obtaining the most perfect delineations of objects.