. The evolution of forces . the light of day,and then jslaced behind the box H. In alew seconds there willappear on its surface the image of the oVjject enclosed in the image can be fixed by placing the screen I for from thirty tosixty seconds on a photographic plate, which is subsequeiitlydeveloped. If it is desired to take a photograph in the dark chamber the screenI should be placed at M in the frame of an ordinary camera, fur-nished with a portrait diaphragm. The focussing sliould be donepreviously. Now let us examine how, by utilising the proper-ties of these radiations, a body e
. The evolution of forces . the light of day,and then jslaced behind the box H. In alew seconds there willappear on its surface the image of the oVjject enclosed in the image can be fixed by placing the screen I for from thirty tosixty seconds on a photographic plate, which is subsequeiitlydeveloped. If it is desired to take a photograph in the dark chamber the screenI should be placed at M in the frame of an ordinary camera, fur-nished with a portrait diaphragm. The focussing sliould be donepreviously. Now let us examine how, by utilising the proper-ties of these radiations, a body enclosed in an opaquecase may be rendered visible. We have simply tofollow the explanations given below the above figure. 304 THE EVOLUTION OF FORCES In a few seconds the object enclosed in a box isseen outlined on the screen Avhich covers it. The luminous source of which the rays passthrough the opaque body is a paraffin lamp coveredwith black paper. The operator is therefore incomplete darkness, in the midst of which there. Fig. 32.—Photograph in a dark room of an object ( adecorntion) shut up in an opaque box. The experiment isarranged as in Fig 31. The index of refraction of lensesfor radiations of great wave-length being unknown, Ihave had to put up with an approximate focussingwhich has made the image not very clear. This photo-graph of an object shut up in an opaque box is the firstto be made with a camera. As the X-rays are notrefrangible, one cannot photograph with them in adark room. appears on the screen the objects contained in theopaque box. Of the various experiments realizable by the aboveprocess, the most striking is that of the visibility ofan object, key, decoration, &c., enclosed in a being placed in front of the invisible lamp, wesee gradually appearing out of the total darkness in PHOTOGRAPHY THROUGH OPAQUE BODIES 305 which the operator is the image of the enclosedobject. When working with transparent screens ofh\rge size the effect is surpris
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