en. LVIIL] VISUAL JUDGMENTS 855 



We have already seen that in spite of the reversion of the image 

 in the retina, the mind sees objects in their proper position; this 

 is explained on p. 823. 



We are also not conscious of the blind spot. This is partly due 

 to the fact that those images which fall on the blind spot of one eye 

 are not focussed there in the other eye. But even when one looks 

 at objects with one eye, there is no blank, for the reason explained 

 on p. 835. 



Our estimate of the size of various objects is based partly on the 

 visual angle (p. 822) under which they are seen, but much more on the 

 estimate we form of their distance. Thus a lofty mountain many 

 miles off may be seen under the same visual angle as a small hill 

 near at hand, but we infer that the former is much the larger 

 object because we know it is much farther off than the hill. Our 

 estimate of distance is, however, 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 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 mind, into which the 

 sensations of vision are incorporated, invests the images of objects, 

 together with the whole field of vision in the retina, with very vary- 

 ing 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 such as 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- 



