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 trans- 

 parent 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 them- 

 selves 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 per- 

 fectly 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 aper- 

 ture of the eye, this is not the fundamental thing in adaptation, but 

 there is some change in the retina which gives it greater sensitive- 

 ness. 



