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Mr. Dancer also read a paper “ On Pseudoscopic Yision 
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 apparently 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 edge is turned towards 
the nose, the surface will appear convex. The full effect of 
this alteration in the appearance of the object is not realized 
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 a bright pattern, 
having a few articles disposed towards the edges, exhibits 
this fallacious vision in a marked degree. 
The angle of the prisms for shewing these experiments 
should be about 15 degrees, 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 degrees, 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 
experiments to Hhose which are uncorrected for colour. 
Experiments with these prisms have shewn that the power 
of converging the optic axes, differ very considerably in 
individuals. 
