INTRODUCTION. 
13 
tremor being thus nullified. We must have either 
synchronous vibration or none. 
The illumination of what is often called the “ dark ” 
room is a matter of consequence, but a suitable 
lamp may be obtained from any photographic dealer. 
Ordinary light, as is well known, affects the sensitive 
salts in the photographic him, but if ordinary light 
be divided into its component parts by means of 
“ spectrum analysis,” it will be found that, though the 
whole of the visible spectrum affects the sensitive salts, 
those rays which are nearest to the violet end of the 
spectrum affect the plate much more than do the rays 
more nearly approaching the red end of the spectrum. 
In fact, it may be taken that in practice the red 
and orange parts of the spectrum do not affect an 
ordinary plate to any appreciable extent, unless the 
plate is exposed to such rays for a very long time—for 
a longer time than is usually required for the “ develop¬ 
ment ” of the plate. 
We are able, in consequence, to watch the progress 
of development by using red or orange, or even yellow, 
light in the room where the operation is carried on. 
But if the light from the lamp, or from the sun, is 
allowed to reach the plate without the chemically 
active rays having been stopped, the plate will certainly 
suffer. Accordingly, we use, to filter out the “ actinic,” 
or chemically active, rays, some medium, such as 
transparent ruby glass, or translucent, green-yellow 
paper, or other fabric. Workers who use ordinary 
plates generally prefer the yellow fabrics, but for our 
