76 
IOWA ACADEMY OF SCIENCES. 
shadows and pink in the lights. Similarly, the latter 
picture will be invisible to the eye covered with the green 
glass, but the pink picture will appear to this eye as a 
picture black in the shadows and green in the lights. 
Thus the condition for stereoscopic vision, which is that 
the one eye shall see only the one picture and the other 
eye only the other picture, is fulfilled. It might seem at 
first thought that the differences in the pictures due to the 
fact that the negatives were taken through color- 
screens might cause a difficulty, but experiment shows 
that the relief is perfect. The prints should be made to 
register in the background. 
Now, since certain colors are prevented from entering 
each eye it follows, of course, that each eye alone can not 
see a natural color picture, but since the one filter trans- 
mits just what the other stops, the brain will get the 
correct colors provided that it can combine the color sen- 
sations. Although ordinarily when we attempt to combine 
in the brain color sensations received by the two eyes there 
occurs what is called color rivalry, there being a predom- 
inance first of one and then of the other color, yet under 
the conditions here described, where the attention is fixed 
by a picture, the brain seems to have the power of making 
the combination. At any rate, experiment shows that 
when the viewing glasses are of the proper relative density, 
then the picture is seen not only in bold relief, but also in 
its natural colors. 
Such a stereoscopic transparency also shows well when 
viewed through the colored spectacles directly, instead of 
being projected. Prints on paper similar to the well- 
known “anaglyph” prints could also be made. 
Evidently, instead of using green and pink for the spec- 
tacles we can use either of the other primary colors 
together with its complementary, namely, red and cyan- 
blue, or blue-violet and yellow, the negative taken from 
the displaced point of view being in the former case the 
red-record negative, so that the cyan-blue print is the odd 
one, and in the latter case the blue-violet record negative, 
so that the yellow print is the odd one. 
