PHOTOGRAPHIC PLATES. 
99 
selecting the illuminant for the developing room—in 
fact, we must either work in almost total “red darkness,” 
or obtain from the firm glass specially adapted for the 
purpose, the latter course being decidedly the better. 
It is impossible here to go into all the possibilities of 
these plates; but a few may be mentioned. For the 
dense yellows found in some insect preparations, and 
for such sections as we have seen of bone stained 
deeply with rubin, and for other very puzzling single 
and double stains, these plates are to-day almost a 
necessity. They ought to be used with suitable 
screens, which must be chosen with due knowledge 
and consideration of the colours and combinations of 
colours in the preparation, but one very generally useful 
screen is that made by the same firm and called the 
“ Gilvus ”—this increases the exposure necessary about 
six times—-while a screen even more selective, and in 
some cases even more useful, if not essential, is the 
“ Absolutus,” which increases the exposure perhaps 
forty times. The sensitiveness of these plates without 
a screen is very great, but they do not yield great 
density—we allude to the medium rapidity—so that we 
do not use them for everyday work. 
It is highly advisable to use all kinds of plates 
“ backed,” especially very sensitive ones. The rays of 
light are projected right through the emulsion to the 
back of the glass plate, and from that surface are 
reflected back to the sensitive emulsion at such angles 
that they produce blurring in the image; if the back of 
the plate is coated with some substance of non-actinic 
