200 Orthochromatic Photography. [Sess. 
be observed that with the yellow screen the curve is actually 
higher in the yellow, and extends farther into the red. This 
is owing to the longer exposure, which gives these colours 
more time to affect the film. You will remember that we got 
something like this effect even with an ordinary plate, but in 
the case before us the exposure was very much less in dura- 
tion, owing to the plate having been orthochromatised with 
eosin. Here is a photograph of the spectrum on a Cadett 
spectrum plate which has been exposed to the spectrum, a 
yellow glass being interposed. The glass was not quite of 
the right tint, as we can see that the blue about the A line 
might have been reduced a little more with advantage (Plate 
XXIIT.: 9). 
We are now in a position to apply the principles which we 
have established as regards orthochromatic photography. It 
will be obvious that its most useful application is in the re- 
production of objects more or less definitely coloured. For 
this work a good brand of plates is essential. Those prepared 
by Lumiére Brothers of Lyons, and marked A, B, and OC, are 
excellent. The Cadett spectrum plate is a remarkably good 
plate, and has also the valuable quality of rapidity—I judge, 
about four times that of the Lumiére plates. The Ilford 
chromatic is a good plate for ordinary work, but slow. The 
Edwards plate gives very good results. Rapidity in an ortho- 
chromatic plate, especially for outdoor work, is of the utmost 
value, because the use of the tinted light-filters prolongs the 
exposure. With a good rapid plate and a lens that works 
well at open apertures, the difficulty of longer exposure can 
be fairly overcome. 
Tinted ray-filters are necessary if good results are desired. 
These can be purchased of various intensities, made out of 
optically worked glass, for a reasonable price. The prevailing 
tint is yellowish. I strongly recommend using the filters 
advocated by the makers of the plates; they are usually 
specially prepared to suit the various brands. Cadett & 
Neal are particular in calling attention to their light-filters, 
and though I have not yet used them I intend doing so. 
Much judgment is required as to what grade of ray-filter shall 
be used. Suppose we wished to take a landscape with a blue 
sky having many light white clouds drifting. In an ordinary 
