810 THE EYE AND VISION [CH. LVI. 



estimate of distance is often erroneous, and consequently the 

 estimate of size also. Thus persons seen walking on the top of 

 a small hill against a clear twilight sky appear unusually large ; 

 because we over-estimate their distance, and for similar reasons 

 most objects in a fog appear immensely magnified. The same mental 

 process gives rise to the idea of depth in the field of vision ; this 

 idea is fixed in our mind principally by the circumstance that, as 

 we ourselves move forwards, different images in succession become 

 depicted on our retina, so that we seem to pass between these images, 

 which to the mind is the same thing as passing between the objects 

 themselves. 



The action of the sense of vision in relation to external objects 

 is, therefore, quite different from that of the sense of touch. The 

 objects of the latter sense are immediately present to it; and 

 our own body, with which they come in contact, is the measure of 

 their size. The part of a table touched by the hand appears as large 

 as the part of the hand receiving an impression from it, for the part 

 of our body in which a sensation is excited, is here the measure by 

 which we judge of the magnitude of the object. In the sense of 

 vision, on the contrary, the images of objects are mere fractions of 

 the objects themselves, realised upon the retina, the extent of which 

 remains constantly the same. But the imagination, which analyses 

 the sensations of vision, invests the images of objects, together with 

 the whole field of vision in the retina, with very varying dimensions ; 

 the relative size of the image in proportion to the whole field of 

 vision, or of the affected parts of the retina to the whole retina, 

 alone remains unaltered. 



The estimation of the form of bodies by sight is the result partly 

 of the mere sensation, and partly of the association of ideas. Since 

 the form of the images perceived by the retina depends wholly on 

 the outline of the part of the retina affected, the sensation alone is 

 adequate to the distinction of superficial forms from each other, as 

 of a square from a circle. But the idea of a solid body like a sphere, 

 or a cube, can only be attained by the action of the mind construct- 

 ing it from the different superficial images seen in different positions 

 of the eye with regard to the object, and, as shown by Wheatstone 

 and illustrated in the stereoscope, from two different perspective pro- 

 jections of the object being presented simultaneously to the mind by 

 the two eyes. 



Thus, if a cube is held at a moderate distance before the eyes, 

 and viewed with each eye successively while the head is kept 

 perfectly steady, A (fig. 601) will be the picture presented to the 

 right eye, and B that seen by the left eye. Wheatstone has shown 

 that on this circumstance depends in a great measure our conviction 

 of the solidity of an object, or of its projection in relief. If different 



