194 Orthochromatic Photography. [Sess, 
proportions in the visible part, and also produced at each 
end to show the extent of the invisible rays—ze., the infra- — 
red and the ultra-violet. (The lines marked A, B, C, D, E, 
F, G, H are the Frauenhofer lines.) The upper curve re- 
presents the intensity of luminosity at any part of the visible 
spectrum. It is highest, as we see, in the yellow, and falls — 
away rapidly at either side till it becomes quite faint in the 
blue, indigo, and violet, and also in the extreme red. Now, 
in order to represent these colours in monochrome, say as in © 
an ordinary wash drawing, the bright yellow must be nearly 
white; the orange as a grey turning to darkish grey in the 
bright red and nearly black in the deepest red; the greens 
from light grey to darkish grey in the blue to dark grey, — 
nearly black, in the indigo and violet. You will observe that — 
it is impossible to represent difference in colowr tint in mono- — 
chrome, and that it is only the difference in luminosity that 
can be so represented. For instance, a dark blue and a dark 
red, though so different in colour, could only be translated, 
each of them, into a dark grey. A light green and an orange 
of the same degree of luminosity could not be represented — 
otherwise than by the same light grey tint in a wash drawing. — 
From all this we can easily infer that if a photographic plate 
is to give us a proper rendering in monochrome of the colours — 
of the objects photographed, the amount of deposit of silver in 
the film would be correctly represented by a curve which ~ 
coincides with that of the luminosity of the spectrum. But — 
what do we find on actual trial? This lower curve gives the 
answer. It shows the amount of deposit on the photographic 
film caused by the different coloured rays to which it has — 
been exposed. There is practically no deposit under the 
yellow, the orange, and the red rays, so that these colours, 
especially the yellow and orange, which constitute the brightest — 
part of the spectrum, are photographically represented as 
blacks. When we look at the other end of the spectrum — 
we find under the very much less luminous rays, such as the 
blue, indigo, and violet, the plate has been powerfully affected, 
and the deposit very dense—so much so that these colours, 
as is well known, are generally represented as white by 
photography. In addition to this, we see that even beyond 
the visible limit of the spectrum there are rays that can 
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