MENTAL APPBECIATION OP 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 retinas, 

 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 P 2 



