ZOOLOGY AND BOTANY, MICROSCOPY, ETO. 
115 
of objects coloured or uncoloured. It is necessary, however, that the 
focus and the exposure should be made in the same monochromatic 
light, corresponding to a determined spectrum colour ; this method of 
working assures the perfect sharpness of the image, inasmuch as the 
chemical focus is corrected. For this reason it is desirable to avoid, 
in exposing on one object, the use of screens of two different colours, 
except in the case where, on account of the presence of a deep red, it 
would be useful to prolong the exposure. The plates, after treatment 
with erythrosine and cyanine, being sensitive to the red and bluo it is 
indispensable, to give the image its maximum of effect, to illuminate 
the object with a yellow or orange light, arresting or moderating the 
action of the actinic rays, and leaving free to pass the radiations corre- 
sponding to the colour of the object, if that is green, yellow, orange, or 
red. 
The following table will give an idea of the employment of coloured 
screens with plates sensitized with cyanine and erythrosine : — 
Object in Monochrome — Actinic Rays. 
Colour. Sensitizer. 
Blues or violets j • • Erythrosine .. 
Chemical Rays . 
Greens 1 
Yellows > Erythrosine .. 
Yellow-orange ) 
Orange-red . . ... . . j 
Red > Cyanine 
Deep red j 
Screen. 
Pale yellow. 
Deep yellow or orange. 
Deep yellow or orange. 
I Deep yellow or orange. 
\Orange-red. 
Green and yellow 
Green and red .. 
Yellow and red 
Green and red . . 
Yellow and red 
Coloured Objects — Non- Actinic Colours. 
Erythrosine . . . . Deep yellow. 
• • | Erythrosine .. .. | D ee^yellow or orange, then 
' ' | Erythrosine and cyanine Deep yellow or orange. 
Actinic Colours in the Presence of Non- Actinic Colours. 
Blue or violet with yellow. Erythrosine Light or deep yellow, or 
orange, according to the 
intensity of the blue or 
violet. 
Bine or violet with red . . Cyanine Same screens ; in case red 
is very deep continue the 
exposure with a red 
screen. 
At first sight it might seem abnormal to attempt to photograph an 
object of a blue or violet colour with an emulsion sensitized for yellow, 
but M. Monpillard says nothing is more rational. It will suffice to 
obtain an image showing vigorously, with the shadows and half tones 
well rendered. Now, with an emulsion which is particularly sensitive 
to the blue and violet, ifche object in question will appear so luminous 
that the ground will be slightly lost, and the half tones will not come 
up well. The interposition of a yellow or orange screen will retard the 
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