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I yo PROJECTION OF IMAGES OF OPAQUE OBJECTS [Cn. .VII
passing through a hole, or later a lens, in the wall of a dark room sufficed to produce the picture on the white wall or screen.
Later it was found that it was possible to illuminate objects sufficiently with artificial light to get screen pictures; and still later transparencies were used (§ 272a).
Every one who looks at the picture of a landscape, etc., depicted on the ground glass of a photographic camera sees inverted images like those originally observed in darkened rooms on translucent screens.
CONDITIONS FOR OPAQUE PROJECTION: COMPARISON OF PROJECTION WITH OPAQUE AND TRANSPARENT OBJECTS
§ 273. In order to deal intelligently and successfully with opaque projection it is necessary to comprehend in the very beginning the difference in the conditions for obtaining a screen image of an opaque object, and for a screen image of a transparency (lantern slide, moving picture film or microscopic specimen).
With a transparent or semi-transparent object, the light comes from behind and traverses the object, and goes on with practically no variation in direction to the projection objective. As the light reaching the lantern slide or transparency is directed by the condenser (fig. 91), the light which illuminates the transparency passes on and enters the projection objective and therefore serves for the production of the screen image (fig. 1-2).
With the opaque object, on the other hand, all the light which produces the screen image must be reflected from the surface of the object, and the light which illuminates the object must strike its
§ 272a. In the early days of opaque projection with artificial light the whole face of a man was sometimes shown; this, of course, required very large lenses.
This is what Hepworth says concerning these exhibitions: "At one time a large instrument of this type was made for casting the image of a human face on the screen, the lenses being of immense size. . . It was, of course, fitted with a reversing (erecting) lens (fig. 208), so that the face should appear right way up. The owner of this face, by the way, suffered tortures during the short time of exhibition, for the powerful lime lights close to and on each side of his head, were so hot that they blistered his skin. He was made to smile at the audience, and then to drink their good health in a glass of wine, a refreshment which the poor man really needed after his grilling." (P. 246).