Exposure 

working backwards with the actinograph  taking a thoroughly satisfactory exposure as the basis  some approximation towards the determination of a number for any given batch of plates may be made, even by the casual worker. More or less arbitrary speed numbers are used with the various actinometers, and owing to the very different conditions under which the estimates are made, and the fact that the speed of a plate is rather a complex matter than a simple quantity, these numbers are by no means comparable among themselves. Another difficulty in comparing various scales arises from the fact that the degrees progress unequally on some scales, although in the case of the Hurter and Driffield, and perhaps some others, the sensitiveness is in direct ratio with the numbers, so that a plate corresponding to, say, io^{o} will not be precisely twice as sensitive as one corresponding to 5^{o}. This is very well illustrated by the following table of the experimental value of the degrees on Warnerke's sensitometer, one of the first instruments which came into general use for determining the relative rapidities of plates :  

Cadett's Table, showing the relative Rapidities of
Plates of varying Sensitometer Numbers. Number of times more sensitive than 
25 24 23 22 21 20 19 18 17 16 15 B 







300 
