The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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The first three, a, &, c, which are thicker at the centre than at the margins, are convergent or positive; and the second three, d, e,ff are divergent or negative. All lenses may, for theoretical purposes, be regarded as formed by the union of prisms, and therefore have to a great extent the properties of prisms. Fig. 72 will show the way in which the prisms are united to form a bi-convex lens, and the concentration of the rays of light by such
Fig. 72.
prisms or lens. Fig. 73 is the representation of the prisms form-ing a double-concave, and the divergent action of such prisms or lens upon the rays of light. It is obvious that by combining the two lenses the convergent or positive action of the one may be counteracted by the divergent or negative action of the other. It is upon these principles that the whole of the modern lenses are calculated. As soon as it was announced in 1830 by
Fig. 73-
Daguerre and Fox Talbot that they had been enabled to obtain a comparatively permanent image in the camera obscura, the ability and skill of mathematicians and opticians were brought into play to produce lenses which should be free from the objections common to the double-convex lens, which was that generally used in the camera obscura in those days. The single lens was replaced by the achromatic combination of the telescope (fig. 74). This was eventually reversed, and the plane side presented to