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108 THE ADVANCE OF PHOTOGRAPHY
verge in one point, the rays from A in a, and those issuing from B in b ; and the result is that a perfect miniature and inverted image of the arrow is produced.
If the arrow be moved near to the lens, its image is removed farther from the lens and becomes larger. For example, if the little arrow a b is placed before the lens, it produces the enlarged picture A B.
But if the arrow be removed farther from the lens, its image approaches the lens, and becomes continually smaller. Accordingly, a lens is able to project enlarged or diminished images of an object, the size of the image varying with the distance of the object from the lens.
Different Classes of Spherical Lenses. — So far we have only been considering lenses both faces of which are convex. The lenses used for optical purposes are known as " spherical " lenses, since their faces are either plane or parts of a sphere. There are, however, lenses made which have cylindrical, elliptical, and even parabolical curved faces.
Of the spherical lenses there are six kinds, as shown in the following diagram (Fig. 32).
( I. is uouuie convex. iiv.
) ii. ,, piano convex. t> ) v. ,, piano concave.
) in. ,, concavo convex (con *)vi. ,, concavo convex (diverg
is double convex. t iv. is double concave.
l. ,, piano convex. t>
verging). ( ing).
ni. and vi. are often called meniscus lenses.
The class A are all thicker at the centre than at the edges, and parallel rays of light falling upon one side of any of these will converge on the other side towards a point on their axes, hence these are called converging lenses. The Class B are all thinner at the centre than at the edge, and parallel rays falling on these will diverge on the other side, hence these are known as diverging lenses.
In order to understand some of the difficulties which beset the pioneers in the manufacture of photographic lenses, it will be necessary to deal in a little more detail