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 
through a mental process, based on the consentaneous percep¬ 
tion of the two dissimilar pictures formed on the two retinae, 
that these are made to blend into one representation, which 
gives the idea of '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 
rise 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. 1 In looking with a single eye, 
1 In making these and similar experiments, it is necessary to take 
care that the whole of the projecting or receding surface is equally 
illuminated; since the presence of any shadow proceeding from a 
known source of light, destroys the illusion, by forcing the mind to 
recognise the real figure of the object. 
F F 2 
