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190 PROJECTION OF IMAGES OF OPAQUE OBJECTS [CH. VII
§ 292. Avoidance of shadows. — With solid objects there will be very heavy shadows unless the light is evenly distributed. With a single lamp this is not easily accomplished, and if no mirror is used practically impossible. It is better to use two lamps, one on each side, as in the original apparatus of Chadburn (fig. 93). The two lamps have the further advantage of doubling the light. Two arc lamps are used in the large opaque lantern of the Bausch & Lomb Opt. Co. (fig. 107).
In the Spencer Lens Co.'s opaque lantern, plane mirrors line a part of the projection chamber where the object is placed, and much of the light lost by absorption without this arrangement is reflected back upon the object. This also helps to obviate the shadows when one lamp is used (fig. 1 1 1).
ERECT IMAGES WITH OPAQUE OBJECTS
§ 293. Inversion of the image with an opaque object. — Besides being upside down the image of an opaque object on an ordinary white screen has the rights and lefts reversed.
§ 294. How to get an erect image with the object in a vertical position. — Put the opaque object in the vertical position upside down. Point the objective at right angles to the screen, use a mirror at 45 degrees, or use a 45 degree prism to direct the imageforming rays upon the vertical opaque screen (fig. 95, in). If the inversion of the rights and lefts is unimportant, put the object upside down in the vertical holder and point the objective directly toward the screen (fig. 97, 109).
If a translucent screen like ground glass is used the image will be erect in every way if it is put upside down in the holder and the objective pointed directly toward the screen.
§ 295. How to get an erect image of an opaque object in a horizontal position. — Place the opaque object with its upper edge away from the screen. The objective is usually in a vertical position so that the image would appear on the ceiling above the instrument. The mirror or prism used to direct the image forming rays upon the vertical screen corrects also the mirror image, and