Читать книгу A Practical Manual of the Collodion Process. Giving in Detail a Method For Producing Positive and Negative Pictures on Glass and Paper онлайн

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Spherical aberration is overcome by proper care in the formation of the lens:—"It can be shown upon mathematical data that a lens similar to that given in the following diagram—one surface of which is a section of an ellipse, and the other of a circle struck from the furthest of the two foci of that ellipse—produces no aberration.

"At the earliest period of the employment of the camera obscura, a double-convex lens was used to produce the image; but this form was soon abandoned, on account of the spherical aberration so caused. Lenses for the photographic camera are now always ground of a concavo-convex form, or meniscus, which corresponds more nearly to the accompanying diagram."

Fig. 11.

CHROMATIC ABERRATION.

Chromatic Aberration is another difficulty that opticians have to contend with in the manufacturing of lenses. It will be remembered, that in a former page (ssss1) a beam of light is decomposed by passing through a glass prism giving seven distinct colors—red, orange, yellow, green, blue, indigo and violet.