242 



DEVELOPERS AND LIGHT FOR DEVELOPING [Ch. VII 



yellow, and hence the dark-room light must exclude those, or be red 

 only. 



For panchromatic plates which are sensitive to all wave lengths 

 the only safe method is to develop in total darkness for any light 

 will fog the plate if it acts sufficiently upon it. Sometimes very dark 

 green is used, for the eye is most sensitive to green if the light is very 

 dim, although for bright Ught the eye is most sensitive to yellow. 

 But to be able to see clearly enough to determine the stage of devel- • 

 opment by the green hght dim enough to be safe one must be in the 



Fig. 143. Dark Room for Photography and Drawing in a Large Room. 

 (From Optic Projection). 



dark room for half an hour or more. The total darkness method is 

 safest. One learns rather quickly to work in total darkness, and the 

 time during which development goes on can be determined by count- 

 ing seconds or a signal clock ringing minutes or an alarm clock which 

 can be set at the beginning for the estimated time can be used. Or 

 finally, one can develop in a tray which is covered so that no Hght can 

 reach the plate; then the ordinary dark-room Hght can be turned on 

 from time to time to see when the estimated period for development 

 has been reached. 



It is far safer to use too Httle light for developing rather than too 

 much. For ordinary or for isochromatic plates only a brief glance 

 occasionally is all that is needed. If one holds the plate in the dark- 

 room light during the whole development or for a considerable time 

 there is almost always a thin veil of fog which lessens the crispness of 

 the picture. 



