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444 DARKENING THE PROJECTION ROOM [Cn. XII
jection room depends entirely upon the brilliance of the screen image. In order to see the screen image clearly there must be strong contrast between it and surrounding objects. With transparent lantern slides and sunlight or the electric light to illuminate them one can see the screen images well in a room so light that everything in the room is visible provided no direct light reaches the screen except 'that from the projection apparatus. If the lantern slides are less transparent or the light used for projection less brilliant, then the room must be relatively darkened to give the needed contrast. Keeping the principle of contrast in mind, one readily understands that for some of the experiments in physics where the light on the screen is very dim, with kinemacolor moving pictures and with Lumiere colored lantern slides, and with high power micro-projection, the room must be very dark in order to get the screen image clearly visible. In like manner if the source of light for projection is relatively weak, like the acetylene flame or some other less brilliant light than the electric arc, the room must be darker than with a more brilliant radiant.
§ 609. Daylight and twilight vision. — It has been known for time out of mind that with most people the eyes can adapt themselves to a dim light or to a bright light. If one goes into a dimly lighted room from full daylight the room will at first appear perfectly black, but in a few minutes objects can be seen fairly well, and within half an hour the room will appear comparatively light. On the other hand, in passing from a comparatively dark room to full sunlight the eyes are so dazzled at first that hardly anything can be seen, but soon the eyes become adapted to the bright light. It has been found by careful experiments on large numbers of people that the main adaptation of the eyes for bright light after being in a dark room requires only about 6 minutes, while the adaptation for a dim light after being in full daylight requires about 30 minutes, although after 10 minutes the eye is about 100 times as sensitive in a dark room as it is in full daylight. While the pupil expends normally in dim light, thus increasing the aperture of the eye, this is not the fundamental thing in adaptation, but there is some change in the retina which gives it greater sensitiveness.