348 AN AMERICAN TEXT- BO OK OF PHYSIOLOGY. 



ous terminations of the nerves of touch enable us to judge of the size and 

 shape of an object brought in contact with the skin. This spatial perception 

 is materially aided by the muscular sense of the muscles moving the eyeball, 

 for we can obtain a much more accurate idea of the size of an object if 

 we let the eye rest in succession upon its different parts than if we gaze fixedly 

 at a given point upon its surface. The conscious effort associated with a given 

 amount of muscular motion gives, in the case of the eye, a measure of distance 

 similar to that secured by the hand when we move the fingers over the surface 

 of an object to obtain an idea of its size and shape. 



The perception of space by the retina is limited to space in two dimensions 

 — i. e. in a plane perpendicular to the axis of vision. Of the third dimension 

 in space — i. e. of distance from the eye — the retinal image gives us no know- 

 ledge, as may be proved by the study of after-images. If an after-image of 

 any bright object — e. g. a window — be produced upon the retina in the man- 

 ner above described and the eye be then directed to a sheet of paper held in 

 the hand, the object will appear outlined in miniature upon the surface of the 

 paper. If, however, the eye be directed to the ceiling of the room, the object 

 will appear enlarged and at a distance corresponding to that of the surface 

 looked at. 1 Hence one and the same retinal image may, under different cir- 

 cumstances, give rise to the impression of objects at different distances. We 

 must therefore regard the perception of distance not as a direct datum of vision, 

 but, as will be later explained, a matter of visual judgment. 



When objects are of such a shape that their images may be thrown suc- 

 cessively upon the same part of the retina, it is possible to judge of their rela- 

 tive size with considerable accuracy, the retinal surface serving as a scale to 

 which the images are successively applied. When this is not the case, the 

 error of judgment is much greater. We can compare, for instance, the relative 

 length of two vertical or of two horizontal lines with a good deal of precision, 

 but in comparing a vertical with a horizontal line we are liable to make a con- 

 siderable error. Thus it is difficult to realize that the vertical and the hori- 

 zontal lines in Figure 158 are of the same length. The error consists in an 



over-estimation of the length of the vertical 

 lines relatively to horizontal ones, and appears to 

 depend, in part at any rate, upon the small size 

 of the superior rectus muscle relatively to the 

 other muscles of the eye. The difference amounts 

 to 30-45 per cent, in weight and 40-53 per cent, 

 in area of cross section. It is evident, therefore, 

 that a given motion of the eye in the upward 

 direction will require a more powerful contraction 

 of the weaker muscle concerned in the movement 



fig. 108.-T0 illustrate the over-esti- than will be demanded of the stronger muscles 



mation of vertical lines. , in i 



moving the eye laterally to an equal amount. 



1 This power of the surface of projection to determine the apparent size and distance of the 

 after-image may be to some extent influenced by the will. — Jeffries : Journal of Boston Society 

 of Medical Scit nces, vol. i. No. 9. 



