CH. XII] 



SCREENS WITH METALLIC FACIN G 



459 



image much more brilliantly illuminated than those at the side. 

 Sir David Brewster in 1832 advocated and used the bright metallic 

 surface on the back of looking glasses, which at that time was 

 composed of mercury and tin. Later, surfaces covered with 

 silver-leaf, silver particles or particles of aluminum have been tried. 

 Last of all, plate glass has been ground on one side, and the smooth 

 side silvered. The ground surface of the glass is turned toward the 

 projection apparatus and facing the spectators who get the image 

 reflected from the mat surface of the glass and transmitted from 

 the mirror through the mat ( 6 2 pa). 



FIG. 246. DISTRIBUTION OF LIGHT 

 REFLECTED FROM A WHITE 



SCREEN. 



It is approximately uniform through- 

 out the entire hemisphere. 



FIG. 247. DISTRIBUTION OF LIGHT 

 FROM A SEMI-DIFFUSELY RE- 

 FLECTING SCREEN. 



The closeness of the arrows indicates 



the apparent brightness as seen 



from different directions. 



630. Suitability of metallic screens. Metallic screens are 

 not suitable for micro-projection, or, indeed, for any projection if 

 fine details are to be studied close to the screen, but details which 

 can be seen at a distance of 2 to 3 meters are very well brought 

 out on the mirror screen, and other metallic screens. In com- 

 paring a mirror screen, an aluminum bronze screen and one of 

 plaster of Paris or cardboard if the image was observed within 

 the narrow angle of 15 degrees to the right or left of the axis, 30 

 in all, the mirror screen was brightest, the aluminum next, and 

 finally the plaster of Paris or cardboard, the screens being in the 

 field at the same time so that the comparison was under identical 



629a. The authors wish to acknowledge their indebtedness to The Motion 

 Picture Screen Company of Shelbyville, Indiana, U. S. A., for their courtesy in 

 sending a sample of their "Mirror Screen" for experiment; to the Bausch & 

 Lomb Optical Company for the loan of the two metallic screens of Zeiss; to 

 the J. H. Gentner Company of Newburgh, N. Y., for samples of Mirroroide; 

 and to other screen manufacturers for courteous answers to inquiries. 



