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 PROJEC- 

 TION WITH OPAQUE AND TRANSPARENT OBJECTS 



273. In order to deal intelligently and successfully with 

 opaque projection it is necessary to comprehend in the very begin- 

 ning 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 con- 

 denser (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). 



