PHOTOMICROGRAPHS AND LANTERN SLIDES 
149 
well as the Commercial Ortho, has the additional advantage of being 
sensitive to red, but is more difficult to handle because it is fogged 
by the usual red darkroom lamps. Panchromatic films, like panchro¬ 
matic plates, should be developed in an unlighted darkroom, or if 
it seems necessary occasionally to examine the negative during 
development, the special Wratten safelight made for this purpose 
should be employed. The Eastman Kodak Company publishes 
an interesting booklet entitled “Professional Film” which they 
distribute free of charge. 
With fast plates and without filters, a strong light will allow 
exposures of a small fraction of a second but we have had no success 
under such conditions. The ordinary yellow filters used in out-of- 
door work will be worth trying, but if one is contemplating doing 
any considerable amount of serious work we would suggest procuring 
a set of filters made up especially for photomicrography. The set of 
9 filters known as the Wratten “M” filters made by the Eastman 
Kodak Company is very useful. With these filters used singly and 
in combination it is possible to utilize selected portions of the spectrum 
and thus to increase or decrease the contrast either between the 
specimen and background or between the variously stained portions 
of the specimen. For the beginner, perhaps the most confusing part 
of the procedure is in choosing the proper filter. The yellow and 
orange filters will probably be the ones most frequently used, for 
when they are used with a good orthochromatic or panchromatic 
plate or film the result will be an approximately correct rendering of 
the color values. Some workers rarely use any filters except the yellow 
and orange but very frequently other filters will give better results. 
It is not possible to state just which filters will be best for each 
set of conditions but certain rules will aid in determining. To obtain 
the greatest amount of contrast between the specimen and background 
use a filter or combination of filters which transmits only that portion 
of the spectrum completely absorbed by the specimen. If the result 
is unsatisfactory because detail within the specimen has been sacri¬ 
ficed in favor of contrast between the background and specimen, try 
a filter or combination of filters whose spectral transmission is not 
exactly limited to the part of the spectrum absorbed by the object. 
To eliminate contrast between the object and its background use a 
filter or a combination of filters transmitting that portion of the 
spectrum also transmitted by the object. After a little experience 
