Inexpensive Screen for Monochromatic Light.. By J. W. Gifford. 167 
Monochromatic light in this form is therefore well adapted for 
photography with powers from 1/20 in. to 1/50 in., which may thus 
do work little inferior to that with apochromatics, provided they aro 
of corresponding aperture, and I do not think that these highest 
powers have yet been made apochromatic. 
In conclusion, and although out of place here, I hope you will 
allow me to call attention to an advantage which might be gained by 
the use of malachite-green screens for colour-correct landscape photo- 
graphy. I need not refer at length to the well-known gap which 
remains in the spectrum impressed on a cyanine-bathed plate, and 
which lies between lines E and F, in the case of an unboiled, between 
E and D in the case of a boiled emulsion. By over-exposure this 
gap may be filled up, but to the detriment of the remainder of the 
spectrum. Instead of this, by using a suitable malachite-green 
screen after a preliminary exposure without a screen, the gap may in 
either case be filled without damage to the remainder, and the plate be 
very uniformly impressed by all rays of the solar spectrum from A to H, 
and further, if an ordinary yellow-brown glass screen be used for the 
preliminary exposure, a photographed spectrum very nearly identical 
with the visual spectrum may be obtained. The gap appears to me 
to be due to the colour of the gelatin, which absorbs less light of this 
refrangibility than of any other, the absorption of light by the gelatin 
possibly accentuating its action as a sensitizer. With erythrosin- 
bathed plates the same defect is noticeable, and I have always found 
landscapes taken with these or cyanine plates lacking in the greens. 
Possibly this application might also be useful for reproducing objects 
in their natural colours by the Lippmann method, but I have as yet 
made no experiments in this direction. 
