. The evolution of forces . be used. Themost convenient is Japan varnish, which is poured,to the thickness of two or three millimetres, on themovable diaphragm of the focussing tube, for theback of which is substituted a thin plate of micafixed to the sides by strong glue. When the varnish 1 When these experiments were first published I gave away thelittle material enabling them to be repeated to all the learned menwho asked me for them. A distinguished professor of ph3sics,M. Izaru, wrote to me : I hastened to effect the experiment withthe material you were good enough to send me, and I was s
. The evolution of forces . be used. Themost convenient is Japan varnish, which is poured,to the thickness of two or three millimetres, on themovable diaphragm of the focussing tube, for theback of which is substituted a thin plate of micafixed to the sides by strong glue. When the varnish 1 When these experiments were first published I gave away thelittle material enabling them to be repeated to all the learned menwho asked me for them. A distinguished professor of ph3sics,M. Izaru, wrote to me : I hastened to effect the experiment withthe material you were good enough to send me, and I was stupe-fied with the clearness of the results. I should never have be-lieved it would be so evident and so rapid. It must be acknow-ledged, however, that the majority of physicists preferred to denythe exactness of my experiments rather than to repeat tried them with the aid of a photographers red lamp,which instantaneously extinguishes the phosphorescence of thesulphide of zinc, and, naturally, observed Fig. of house through opaque body. To face page 307. PHOTOGRAPHY THROUGH OPAQUE BODIES 307 is dry it will be observed, by interposing it betweenthe sun and the eye, that it appears absolutelyopaque. The object to be reproduced on the groundglass of the camera is focussed, and the diaphragmis put in front of the focussing tube. It thusconstitutes an opaque body placed between the lightand the roughened glass. A screen of sulphide of zinc is then illumined bydaylight, and placed in the usual dark slide of thecamera as if it were an ordinary photographic lifting in the ordinary way the shutter of theframe, it is left open before the object to be repro-duced for a period varying with the light. The photograph here given (Fig. 33) is that of ahouse in daylight, and the objective used was aportrait lens. The exposure lasted one minute. The exposure at an end, the frame is closed andtaken into the dark room, whence care has beentaken to eli
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