JuLy 14, 1899. ] 
a quite different direction from the pangen 
hypothesis. Whether we shall succeed in 
finding it is another question. 
Epmunp B. Wrtson. 
CoLUMBIA UNIVERSITY. 
PICTURES IN THREE DIMENSIONS. 
A Cxicagco publishing firm has put on 
the market a series of pictures in which a 
stereoscopic effect is produced by a device 
which seems not to have been used before 
in this country, but which is well known in 
Germany. Two photographs of an object 
are taken at distances apart equal to the 
distance between the eyes, and with ob- 
jectives whose focal lengths are equal to 
the distance of distinct vision—that is, in 
the ordinary manner of making stereoscopic 
pictures. These two pictures are printed in 
two different colors, say redand green, soas 
to nearly but not quite overlap each other, 
and they are then looked at through spec- 
tacles composed of red and green glass. If 
the red picture is to the right and the green 
picture to the left, then the right eye looks 
through a green glass and sees in strong 
black the picture which is printed in red, 
but overlooks the faint green picture by the 
side of it; at the same time the left eye 
looks through a red glass and sees in sharp 
black outlines the picture which is printed 
in green, but not the faint red shadow at 
the side of it. In this way are produced 
the two halves of a stereoscopic impression, 
and a very good illusion of relief is ob- 
tained. 
That the explanation above given is the 
correct one is proved by the fact that the 
images of near objects are plainly farther 
apart than those of distant ones ; that if one 
looks attentively, with the glasses on, one 
can see the shadowy secondary pictures at 
the right and left of the principal one ; that 
by putting on the spectacles wrong side up 
an inversion of the relief is obtained—near 
_ SCIENCE. 
45 
objects look far and far objects look near, so 
far as this is not interfered with by other 
elements of solid vision, as perspective, 
shadows, overlapping, etc., and that, by in- 
verting the picture as well as the spectacles, 
the correct relief is again obtained; and, 
finally, by the fact that when one sees 
single an object in the foreground, one is evi- 
dently not fixating upon the plain of the 
paper, because the title of the picture, in 
plain black lettering below, is then per- 
ceived to be doubled. 
The pictures of this issue are roughly 
made, and while the illusion is very strong 
it is not at all perfect ; the distance between 
a child in the foreground and a building in 
the background will be, for instance, very 
distinct, but the child wili be itself rather 
flat. With better workmanship, this method 
for securing vision in the third dimension 
ought to have an important future. The 
stereoscope has, for some reason, never lent 
itself to the purposes of art; this process, 
which has much less paraphernalia, and 
hence has its mechanicalness much less in 
evidence, may conceivably fill a more im- 
portant role in this respect. However that 
may be, its usefulness for scientific purposes 
ought to be very great. There are count- 
less intricate things which one desires ex- 
tremely to represent in their solidity, and 
which it is unnecessarily hard for the reader 
to catch the bearing of when they can only 
be seen in the flat. Think for a moment 
how great would be the difficulties of the 
student of geometry if he had no more life- 
like representations of his plane triangles 
than he has of his polyhedra and his parallel- 
opipedons, and then imagine the pleasures 
that are in store for him if he has only to 
pick up his red and green spectacles to see 
the figures of solid geometry in all the 
reality which has hitherto existed for him 
only inthe plane! And what rapid progress 
will be made in the imaginings of the 
stereo-chemist when he is given this ma- 
