H 
PRACTICAL PHOTO-MICROGRAPHY. 
work it will be necessary to use ruby-coloured material 
of some sort. 
The reason is this : all the best photo-micrographers 
have found that it is a great advantage to use “ ortho- 
chromatic ” or “colour-correct” plates. Now, by a 
process to be described later, plates are rendered more 
than normally sensitive to the yellow and orange rays 
of the spectrum, and relatively less sensitive than usual 
to the violet and blue rays ; so that a yellow light, or 
even an orange light, in the developing room, would 
not be “safe”; or, in other words, would affect the 
sensitive salts in the orthochromatic film. So the 
photo-micrographer ought to provide himself from 
the start with a ruby lamp ; a yellow light will answer 
when orthochromatic plates are not in use, as for the 
operations of bromide printing and slide making. 
Lamps for the dark room can be obtained with both 
yellow and ruby glasses or fabrics. For the “ Spec¬ 
trum ” plates mentioned later, special illumination is 
necessary. 
The lamp should be large, and, if permanently fixed, 
should be arranged so that the heated air and the 
products of combustion are carried out of the room. 
If gas is used in the lamp, the tap for turning the gas 
up and down should be on the outside of the lamp, and 
a “ regulator ” burner is of great value, as giving at 
any time a standard light whereby the negatives may 
be examined during progress of development. The 
lamp should be at one side of the operator, or should, 
in any case, be arranged so that its light shall not shine 
