78 PHOTOGRAPHIC FACTS AND FORMULAS 
temperature. The variation in time of development due to 
changes in temperature of the solutions is known as the “tem- 
perature co-efficient,” and is calculated for 10° C. (18° F.). 
In practice it will be found convenient to draw up a table for 
a developer for every 2 degrees rise or fall, which is not a 
difficult matter, as one can use the developer factor and a 
table of logarithms. Expose a plate on a landscape, including 
a portion of the sky, and cut into two, or make two exposures. 
It is necessary to have means for warming up the dish, devel- 
oper and graduate, and also a thermometer must be handy. 
The warming apparatus need be nothing elaborate, a deep 
baking tin or a large developing dish. Fill this with water at 
about 32° C. (90° F.), and place the dish, graduate and 
developer in it. As soon as the developer has reached the 
desired temperature, which for convenience may be 24° C. 
(75° F.), and should not be above 27° C. (80° F.), place 
the plate in the dish, flood with the developer, and note the 
time; watch carefully for the first appearance of the image, 
and again note the time. The plate is no longer any use and 
may be thrown away. It is really advisable to place the plate 
in the dish at the same time as this is floated on the warm 
water, so that the plate itself may be raised in temperature. 
It can be easily covered with an opaque card to protect it 
from light. Then treat the second plate in the same way, only 
this time use the developer cold, and note the time of appear- 
ance exactly as before. If the difference in the temperatures 
of the two developers is exactly 10° C., the temperature co- 
efficient is found at once by subtracting the logarithm of the 
lesser time of appearance from the logarithm of the greater — 
time. Should there be a greater or less difference in the 
temperatures than 10 degrees, subtract the logarithm of the 
lesser time from that of the greater time, and divide by the 
difference of temperatures. The result will be the log. factor 
