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a number of parallel lines, taking care that the lines are fine and
of equal thickness, then cutting from this paper two squares,
corresponding in size to those used in the last experiment,
and submitting them in the same manner to the two eyes (Fig. 2.);
under these circumstances the visual images will be permanently
superposed, and the result will be a check, but without alternation.
To meet the experiment with two discs of different colours,
blue for one eye and yellow for the other eye, let a square of red,
not exceeding one inch, be presented to the axis of one eye, and a
disc of black three quarters of an inch in diameter, to the axis of
the other eye; viewed in the stereoscope, the images will be superposed,
the disc being uppermost, but without the slightest tendency to
alternation; on the contrary, they will remain permanently
superposed.(Fig. 3.)
The experiment refering to the combination of two images unequal in magnitude is one of great interest. That two images differing in size should occasion one resultant perception intermediate of the two impressions, is undoubtedly a remarkable phenomenon, and my endeavour in this, as in the previous experiments, has been to ascertain if the results be