Camera Work: A Photographic Quarterly (1904 (Heft 7))

thus be exactly as 1 to 3. Assuming the tip of the nose to be 2 inches
nearer to each lens than the eyes, we find it 46 inches and 70 inches,
respectively, from the 12-inch lens and from the 18-inch lens. It will
consequently be projected on the plates in proportions of 46/16 = 2.875 by the
smaller lens, and of 70/24 = 2.916 by the larger lens, equal to an increase in
scale of 4.16per cent. and of 2.79 per cent., respectively, as compared to the
scale of reproduction of the plane of the eyes. Applying similar com-
parisons to the parts of the head which are further away from the lenses, the
scale of reproduction will naturally become smaller in direct proportion of
the increase of the distances. Assuming the back of the head to be 7 inches
further away than the eyes, we get, as scales, 48+7 / 16 = 3.4375 and 72 + 7 / 24 = 3.29, or
14.56 per cent. decrease in scale for the 12-inch lens, and 9.66 per cent. in
the case of the 18-inch lens. When adding the percentage of increase in
either case to that of the decrease, we will find the percentage of the total
difference of scale with which each lens will have projected its image. This
shows that the 12-inch lens varied 18.72 per cent., and the 18-inch lens,
12.45 per cent. from the front to the back of the head. Such a considerable
difference must of necessity lead to the conclusion that the longer lens is
preferable and will produce a more natural perspective than the shorter one.
Q.E.D. L. J. R. Holst.

A PHOTOGRAPHIC RAY-FILTER
WHICH IS NOT A COLOR-SCREEN.
AN INQUIRY about mounts for color-screens, recently
made in a Fifth Avenue photo-supply shop, led the
shopkeeper, after some hesitation, to “guess” that it
was a “ ray-filter ” holder that was wanted. I have
been in the habit of regarding the terms as inter-
changeable; but the object of this communication is to
call attention to the merits of a ray-filter which is not
a color-screen, because it has no color.
Such a ray-filter is made by coating plate-glass
with a rather thick film of hard gelatineand then
soaking it for a sufficiently long time in a saturated aqueous solution of
esculin. When perfectly dry this screen, although showing none of the
fluorescence and light-scattering properties of the aqueous solution of esculin,
and although " uncolored ” is, under some circumstances, more efficient for
improving the rendering in photography than light color-screens made with
some dyes which have been recommended for the purpose. The reason for
this is that the most " antichromatic,, rays of the photographic spectrum,
the ultra-violet, are completely suppressed by esculin, but pretty freely
transmitted by some of the yellow dyes. This is so far true that even with
some of the orthochromatic plates in the market, a white screen of esculin
may be as efficient as a moderately deep screen of chrysoidine or uranine,


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