Poy Gee SD 
1900-1901. | Orthochromatic Photography. 199 
lines E, F—i.c., about the region of the green rays, This is 
hardly at all apparent in the Cadett spectrum plate, which 
certainly shows a wonderful evenness in the deposit, and is 
really a remarkably fine plate. Though, according to these 
experiments, two of the Lumiere plates appear to push a 
little farther into the red than the Cadett, still the deposit 
on the Cadett plate in this region, if not quite so far, is much 
more dense. It is quite plain that in all cases the blue 
colours still have a predominant effect on the plates, and 
that there is a want, more or less, of sensitiveness in the 
green. We can well see that in order to get correct, or fairly 
correct, results with these plates, the blue rays must be cut 
out to a great extent. 
lf we are to get correct results with any of these plates, we 
must so arrange matters that the deposit in the yellow part of 
the spectrum is most dense, and much lighter in the blue and 
violet. We must, as it were, push back the summit of the 
curve, which still lies in the blue, back to the D line of the 
spectrum in the yellow. We can do no more with the dyes: 
resort must now be had to an artifice, the same as we employed 
in the case of the ordinary dry plate—a properly coloured light- 
filter must be interposed in the path of the rays, of such a tint 
that the red, orange, yellow, and green rays may pass through 
unimpaired in strength, while the blue, indigo, and violet are 
nearly cut out. The upper curve in fig. 4 shows the effect of 
Chae 
Fig. 4.—Lffect of Light-jilter in Photography. 
a, Plate treated with eosin. 4, Plate treated with yellow screen. 
an eosin-dyed plate on the action of the spectrum; the lower 
curve, the action on the same plate when a yellow screen is 
interposed. The summit of the curve is pushed back into the 
yellow, while the blues are much reduced in intensity. It will 
