THE STEREOSCOPE STEREOMONOSCOPE PSEUDOSCOPE. 461 



and the distance between the two eyes of the individual. Proper per- 

 spective outlines of bodies, when viewed through this stereoscope, con- 

 vey to the mind the idea of solidity. The action of the mind in pro- 

 ducing these results, is shown by the fact, that two pictures of similar 

 objects, differing slightly in size from each other, convey to it the 

 idea of an object intermediate in size. Such a combination probably 

 often occurs in ordinary vision ; for many retinal images of the same 

 object, must be of unequal size in the two eyes, as, for example, those 

 of near objects placed at different distances, and in different direc- 

 tions, as regards each eye ; so likewise, in the case of persons, in 

 whom the two eyes have different focal distances, the two images of a 

 given object must be of different dimensions. But objects at a great 

 distance, those lying opposite the middle of the eye, and those at 

 equal distances from the median plane of the visual field, produce 

 images of equal size. When totally different pictures are viewed in 

 the stereoscope, as for example, a man and a horse, the impressions 

 produced are various ; sometimes the two pictures are blended and 

 confused, sometimes they are seen alternately, and sometimes one is 

 neglected, and the other only seen. The brighter picture usually pre- 

 dominates. In the stereomonoscope of Claudet, the separate images 

 of a solid object, are combined, by means of two lenses, on the same 

 part of a screen of ground glass, when, by their coalescence, they 

 produce, in the eye of the observer, the stereoscopic effect. 



Wheatstorie has contrived another instrument, named the pseudo- 

 scope, which, by diminishing, or increasing, the angles at which the 

 rays of light from an object ordinarily enter the eye, reverses the visual 

 relations between the near and distant points of an image. A concave 

 body appears convex, a convex body, concave ; a bust seen from the 

 front, presents the appearance of a deep hollow mask, whilst the inte- 

 rior of a mask looks like a prominent cast. These phenomena are 

 known as conversions of relief ; they also prove the influence of the 

 mind, in the ultimate interpretation of visual effects. 



When we view distant objects, beyond the range of the binocular 

 parallax, the appearances of solidity and relief, and our conclusions 

 concerning these, are arrived at indirectly, or by processes of judg- 

 ment and experience, in the same manner as when one eye alone is 

 used. Persons who have lost one eye in infancy, must acquire their 

 notions of the solidity of objects, by the conjoined use of touch and 

 sight, aided by movements of the head or body. 



The retina conveys to us a distinct sense of locality, dependent on 

 the stimulation of two or more points of its surface ; and these impres- 

 sions, projected outwards in the field of vision, furnish us with the 

 means of recognizing intervals of space. The two first dimensions of 

 space, viz., length and breadth, like those of a solid body, are easily 

 recognized, even by one eye, according to the angles formed by the 

 different lines of visual direction ; but the third dimension of space, 

 viz., that of depth, is more difficult of appreciation, and requires the 

 use of both eyes. 



Our estimation of the linear direction of external objects, and of 

 their relative, or angular, distances from each other, upwards, down- 



