302 PROCEEDINGS OF SOCIETIES, 
M. Heys, in proposing a vote of thanks, said that he was grati- 
fied to find that the important subject of the structure of cotton, 
in which he had himself for some time felt an interest, was now 
being taken up in a manner likely to clear away all doubts. 
Mr. Dancer also read a paper “On Pseudoscopie Vision through 
Prisms.” 
If we look with both eyes at an object, such as the flat top of a 
table, for example, and then interpose a prism between one eye 
and the object, we discover, after a short time, that the portion of 
the surface to which the sight is particularly directed has appar- 
ently changed its distance. If, in trying the experiment, the thin 
edge of the prism is turned inwards to the nose, the flat surface 
will appear concave ; if, on the contrary, the base or thick angle 
is turned towards the nose, the surface will appear convex. The 
full effect of this alteration in the appearance of the object is not 
realised immediately ; some persons see it perfectly in a few 
seconds, others require some "moments of steady gazing before it 
becomes evident to them. 
The character of the surface to which the vision is directed 
exercises some influence in producing the effect. A circular 
table covered with a cloth of bright pattern, having a few articles 
disposed towards the edges, exhibits this fallacious vision in a 
marked degree. 
The angle of the prisms for showing these experiments should 
be about 15°; if less than this, the elevation or depression of 
surface is not sufficient to produce a good effect; if the angle 
is much greater than 15°, many persons are unable to unite the 
refracted image of the prism with the real image seen by the 
other eye. 
Achromatic prisms are much to be preferred in these experi- 
ments to those which are uncorrected for colour. Experiments 
with these prisms have shown that the power of converging the 
optic axes differ very considerably in individuals. 
Oculists occasionally recommend prismatic lenses, mounted in 
spectacles, to assist persons who suffer from insufficiency of the 
recti interni muscles ; it would be interesting to know if those, so 
assisted, have noticed the fallacious appearances which the healthy 
eye can appreciate. The pseudoscopic effects are exaggerated by 
using a prism to each eye, but in most persons this produces at 
painful sensation. 
The explanation of these phenomena, which I offer with some 
hesitation, is based upon the supposition that in binocular vision 
we estimate the distance of an object by the degree of converg- 
ence of the optic axes. In these experiments, when a flat surface 
appears concave by the interposition of the prism, the optic axes 
are made to converge on a point situated behind the real surface, 
and the imagination gradually removes the object to this apparent 
distance. 
When the base of the prism is towards the nose then the fla 
surface becomes conyex; in this case the optic axes cross in front 
