Lummer, O. (Otto), 1860-1925, Thompson, Silvanus P. (Silvanus Phillips), 1851-1916 · London, New York, Macmillan and Co., 1900 · leaf 112
object is itself very bright. This is — even without a dark room — -an excellent way of observing the sun-spots. The writer, when a boy at school more than thirty years ago, used this method to photograph the spots of the sun. Let us now see how telescope principles may be applied to make a… … Fig. 42. — Focal Length of Positive Lens. between the lens A and its focus, as in Fig. 43, it will reduce the amount of convergence, and as in the lower figure, and cause the rays to meet at F' further away. If these converg- ing rays are produced backwards (as shown by the dotted lines) till they…
Photographic optics
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