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 image- 

 forming 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 



