MENTAL APPRECIATION OF PROJECTION. 435. 
_ were made. In the small portable instrument which has of 
late become so extensively popular, the like effect is produced 
_ by a particular arrangement of convex lenses, devised by 
_ Sir David Brewster, which also has the advantage of magnify- 
ing the pictures. 
562. it is, then, by the combination which is effected 
h a mental process, based on the consentaneous percep- 
tion of the two dissimilar pictures formed on the two retinez, 
_ that these are made to blend into one representation, which 
gives the ideavof projection. When we look at a distant 
object, our judgment is based on less positive data, the two 
; pictures being then almost precisely the same ; and hence it 
| is impossible (without moving the head) to distinguish with 
_ certainty’ between a well-painted picture, in which the pro- 
_ portions, lights and shades, &c. are well preserved, and the 
objects it is intended to represent, if we are prevented from 
knowing that it is a picture. Some admirable illusions of 
this kind have been effected in the Diorama. But a slight 
“movement of the head suffices to dispel the doubt; since 
by this’ movement a great change would be effected in the 
“perspective view of a solid object,—a little of the side of a 
' projecting buttress or column being seen, for instance, where 
only the front was perceived before,—whilst the image formed 
by a picture is but slightly affected. The same indecision is 
experienced when we look with a single eye at certain near 
objects, which the mind can apprehend either as projecting or 
as receding, with equal, or nearly equal, readiness ; such, for 
example, as a metal plate stamped-out into a figure which 
stands-forth in relief on one side and is counter-sunk on the 
other. And the idea of the object which is the reverse of 
the reality may present itself most forcibly, if it should 
happen to be the one most familiar to the mind ; thus if we 
look with one eye at the interior of a mask that has been 
coloured to the semblance of a human face, it will seem to 
ise into the likeness of the exterior ; whilst the actual pro- 
jecting surface of the mask will never seem to exhibit the 
concavity of the interior! In looking with a single eye, 
tin these and similar e en e 
are ge that the whole of the aapilanee decote ie ilateade nedte 
uminated; since the presence of any shadow proceeding from a 
mown source of light, destroys the illusion, by forcing the mind to. 
ecognise the real figure of the es 
FF 
