246 THE OPEN BOOK OF NATURE 
that number by forty, and the time thus arrived 
at is that which I allow for complete development, 
when Rodinal is used. Thus, if it takes twenty 
seconds for the image to make its first appearance, 
that number multiplied by forty gives eight hundred 
seconds, or a little over thirteen minutes. I let my 
plate, then, lie in the developer for thirteen minutes, 
then wash and fix it, and am sure of a good result, 
if the exposure has been right and there has been 
no mishap of any kind. Always cover the dish 
while the plate is lying in the developer. More 
plates are failures through under exposure and under 
development than from any other causes. 
Orthochromatic plates should always be used in 
Nature photography ; they give better colour values 
than ordinary plates. I generally use the Imperial 
Company’s Non-Filter Ortho. plates; I find them 
excellent, but they are liable to be “‘ fogged” if 
the dark-room light is too brilliant. They should 
be handled in a very dull red light, and be always 
covered while development is going on. 
The printing process is a matter of personal taste 
and convenience. P.O.P. gives the best results, in 
my own humble opinion. But the gaslight process 
is very convenient and speedy. I have found some 
Self-Toning papers exceedingly useful. 
Nothing pleases me more than a good lantern- 
slide made from a “ Nature ”’ negative. When such 
a slide is exhibited through an optical lantern, details 
appear that are not seen on flat-paper prints. You 
can make lantern-slides quite easily, and you will 
find them exceedingly useful if you want to illus- 
trate a little Nature-talk given for the benefit of 
