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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 throughout the entire hemisphere.
FIG. 247. DISTRIBUTION OF LIGHT FROM A SEMI-DIFFUSELY REFLECTING 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 comparing 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.