506 Proceedings of the Royal Society 
Is?, When we enter two slips of white card-board, one at each side 
of the stereoscope, they affect non-corresponding parts of the retinae, 
as shown in the diagram exhibited, and they appear as two objects . 
2c?, When we push the slips forward till they appear to overlap, 
the overlapping ends appear as one object, because they affect the 
corresponding central points of the two retinae. If we make a mark 
similar in form and size on each slip, but do not approach the slips 
sufficiently near for the marks at once to coalesce, such marks are 
nevertheless frequently observed to glide closer and coalesce. 
This is owing to the natural tendency we have to direct the axes 
of the eyes to the objects examined. This causes the marks to 
affect corresponding points of the retinae, and the marks conse- 
quently coalesce visually and appear as one object. 
3 d. When the slips seem to overlap, the overlapping portion 
appears so greatly increased in brightness that the other parts have 
a tendency either to disappear altogether, or they appear and dis- 
appear at brief intervals, so long as we continue to look at the 
central bright portion. These dim or invisable outlying parts may, 
however, at any time be made to reappear by simply moving the 
card-board once or twice up and down, and thereby exciting the 
attention and the retina. They may also be made to appear by 
winking, by moving the eyes from side to side, or doing anything 
to stimulate the retinae. 
4:th, With regard, again, to the overlapping parts, it is to be 
observed, that though they appear at first sight to form, as it were, 
one single object, yet it is easy to see that this bright part is in 
reality a double picture containing the impression received from 
each eye; and so far as these impressions are not incompatible, but 
may be blended the one with the other, they go to form as it were 
a composite picture, as we know is the case with the figures on the 
usual stereoscopic slides, and as we may prove to be the case by 
making any distinctive marks on the slips of card-board, when these 
marks will appear distinctly visible, as if integral parts of the over- 
lapping portion, though seen by the different eyes. 
5th , Another important circumstance is this. When the slips 
are of different colours, as for instance one slip red and the other 
blue, or one blue and the other yellow, these colours, when caused 
to overlap in the stereoscope, do not produce the intermediate 
