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28 THE BOOK OF THE LANTERN. give a flat field, and the ordinary diameter of the back lens of such a combination will not admit the whole of the rays from the condenser : hence we are robbed of light, and very often of the corners of square pictures into the bargain. Of late years makers have seen these disadvantages, and have produced lantern objectives which, while they are similar in construction to portrait objectives, are made with much larger apertures at the back. Taking Mr. Dallmeyer's lantern objective as the prototype of these, we find that it consists, like the portrait lens, of two combinations. The back one, next the light, is a convexo-concave of flint, and another of crown glass, separated by a short interval, the two glasses being dissimilar in their curvature. The external form of this combination is a meniscus, its convex surface being next the condenser. The front combination, of smaller diameter, also has the external meniscus form, but consists, like its fellow, of two glasses. It must be noted that objectives made for lantern work are not suitable for photography, for the visual and chemical rays are not coincident. I mention this for the sake of those who wish to use the lantern as an aid to their photo- graphic pursuits, in enlarging and so on. It may^therefore, be in some cases desirable for the purchaser to obtain a half-plate portrait lens for his lantern. It will do excellent work, with the limitation already referred to, while at the same time it can be used either for portraiture or for enlarging. For both these uses it is well adapted. The rule for calculating the distance of the lantern from the screen in order to obtain an image of a given size will