186 
1. The color method of printing stereoscopic 
pictures was invented by L. D. du Hauron, of 
Algiers. Two blocks, e. g., half-tone plates, 
are made from a pair of pictures taken in the 
usual way with the stereoscopic camera. The 
picture taken with the right-hand lens is printed 
in red ink on paper, that with the left-hand 
lens in blue ink directly over it. The result is 
a blurred picture. * When this blur is viewed 
through a pair of spectacles consisting of blue 
glass for the right eye and red glass for the left 
eye the two pictures reach the eyes separately 
and appropriately. This occurs because to the 
eye looking through the blue glass the white 
paper and the blue printing appear—practically 
—an even blue background, while the red ap- 
pears as a black picture ; similarly, to the eye 
with red glass the blue print appears as a black 
picture on ared ground. These two pictures, 
reaching the brain separately, are there com- 
bined into a picture in three dimensions show- 
ing apparently a solid view in wavering pur- 
ple light. The peculiar wavering light is the 
result of fluctuating binocular mixture and 
binocular strife. 
The pictures have been sold as ‘anaglyphs’ 
for a number of years by the Comptoir Suisse 
de Photographie at Geneva. A few years ago 
they were sold by a Philadelphia agent and 
were marked ‘ Patent 8,20,95.’ This is the 
patent-grant 544,666 of August 8, 1895, which 
states that the article was patented in France 
in 1891. 
A peculiar effect arises from twisting the 
picture or the head while observing these pic- 
tures ; the objects in the relief figure appear to 
move relatively to each other. 
2. The projection of stereoscopic pictures by 
a double lantern is not so unknown in America 
as Mrs. Franklin supposes. In the fall of 1895 
I had occasion to deliver a public lecture on 
vision and, not knowing how to do anything 
with binocular vision without some such 
method, I hit upon the idea of throwing the 
two parts of stereoscopic views on the screen in 
red and green lights and giving bits of red and 
green glass to the audience. The method 
proved a complete success at a lecture in the 
Brooklyn Institute. Since then it has been in 
regular use in my laboratory for studying the 
SCIENCE. 
[N.S. Vou. X. No. 241. 
laws of binocular vision. I have, however, no 
claims to credit for the fundamental idea of the 
method. Some photographer seems to have 
previously projected two pictures in a similar 
way ; the details of his process have not been 
accessible tome. The whole method was de- 
scribed and various technical hints were given 
in the Scientific American, 1895, LXXIII., 327. 
This method is now used by several colleges. 
A Philadelphia firm (Williams, Brown & Earle) 
is preparing to furnish the materials and a 
carefully selected set of slides to illustrate scien- 
tific and educational subjects. The equipment 
is so inexpensive for any one possessing a 
double lantern that the cost is hardly worth 
considering. 
3. There is still another method of stereo- 
scopic projection which is in some ways 
superior to the red-green method. The two 
pictures are thrown by two beams of white 
light, polarized at right angles, on to a 
corrugated silvered screen and are viewed by 
an eye glass composed of two analyzers at 
right-angles. As the inventor of this method, 
John Anderton, of Birmingham, England, was 
kind enough to donate a complete outfit to 
the Yale Laboratory, we have had the oppor- 
tunity to use it regularly for instruction. The 
method is superior to the red-green method 
in projecting both pictures in white light, and 
the cost is not excessive. A complete descrip- 
tion of it has been given in my ‘New Psychol- 
ogy,’ p. 4238. 
4, It may be technically justifiable to call 
the results of stereoscopic union by the term 
‘pictures in three dimensions,’ but it is psy- 
chologically incorrect. The view seen by 
stereoscopic projection is—to the observer—the 
real thing. There is no picture effect about it 
when the thing is properly done; the relief and 
solidity of the objects appear just as real as in 
the case of real objects. Of course, it is phys- 
ically impossible to have a group of exotic palms 
where the screen and lecture table were stand- 
ing a moment ago, and the group seen lacks 
coloring. These factors give a slight unreality 
to such a stereoscopic view ; the observer feels 
as though he were looking at a model. In the 
ease of statuary or other objects where color is 
lacking or subordinate the reality is perfect ; to 
