Ch. VII] TIME DEVELOPMENT IN PHOTOGRAPHY 243 



The wisdom of the advice to develop isochromatic or ordinary 

 plates with as small an exposure to the dark-room light as possible 

 can be demonstrated by the beginner in the following experiment 

 which he is advised to try. 



Put an isochromatic or orthochromatic plate in the plate holder. 

 PuU out the dark sUde till one or two centimeters of the film is ex- 

 posed, then leave this for half a minute, close to the developing-room 

 light. Pull out the slide another centimeter or two and expose 

 again to the dark-room light. Continue till the entire plate has been 

 exposed. The last segment will have an exposure of half a minute, 

 next to the last a whole minute, and so on. Now develop the picture 

 in the ordinary way and the chances are that the plate will show 

 very marked light effects, and the different segments in proportion 

 to the time they were exposed to the dark-room light. 



§ 379. Time development. — Assuming that the correct plate 

 and color screen is used, careful experiments made in the scientific 

 laboratories of the large plate manufacturers have shown that the 

 best method of developing photographic negatives is that of devel- 

 oping a definite time at a definite temperature of the developer. The 

 time and temperature must of course be determined for the special 

 plate and composition of developer to be used. The variable then 

 is the exposure of the plate. A perfectly timed plate will contain 

 aU the desired detail in the shadows and just suflScient density in 

 the high lights so that the print will be sufiiciently white. The 

 deepest shadows in such a negative will be almost perfectly 

 transparent. 



A convenient and safe method of developing plates by the time 

 method without having the room absplutely dark and without expos- 

 ing the plate to any harmful light, is the following: The dark-room 

 safehght is directed away from the developing tray and a shield put 

 in position to further screen it. An alarm or other large-faced clock, 

 with second ha.nd, is put close to the safelight. This light may then be 

 very dim and still illuminate the clock face sufficiently. If using 

 isochromatic or orthochromatic plates the red safelight is good, 

 but if panchromatic or spectrum plates are used the green safelight 

 is better. The exceedingly minute amount of light reaching the 



