8o6 



A MANUAL OF PHYSIOLOGY 



identical points. The following is a partial explanation : (i) The 

 images of objects in the portion of the field most distinctly seen, that 

 is, the portion in the immediate neighbourhood of the intersection 

 of the visual lines, or the part to which the gaze is directed, are 

 formed on identical points ; and by rapid movements the eyes fix 

 successively different parts of the field of view. (2) Vision grows 

 less distinct as we pass out from the centre of the retina, and we are 

 accustomed to neglect the blurred peripheral images in comparison 

 with those formed on the fovea. (3) When the images of an object 

 do not fall on identical points, one of the points on which they do 

 fall may be occupied with the images of other objects, some of 

 which may be so boldly marked as to enter into conflict with the 

 extra image and to suppress it. (4) And, lastly, the physiological 

 * identical point ' is not a geometrical point, but an area which 

 increases in size in the more peripheral zones of the retina, so that 



images which lie wholly or in chief 

 part within two corresponding areas 

 practically coincide. 



Stereoscopic Vision. Although 

 the retinal image is a projection of 

 external objects on a surface, we 

 perceive not only the length and 

 breadth, but also the depth or 

 solidity of the things we look at. 

 When we look directly at the front 

 of a building, the impression as 

 to its form is the same whether one 

 or both eyes be used, although 

 with a single eye its distance cannot 

 be judged so accurately. But when 

 we view the building from such a 

 position that one of the corners is 

 visible, we obtain a more correct 

 impression of its depth with the two 

 eyes. This is partly due to the fact 

 that to fix points at different dis- 

 tances from the eyes the visual lines 

 must be made to converge more or 

 less, and of the amount of this con- 

 vergence we are conscious through 

 the contraction of the muscles which 

 regulate it. But there is another 

 element involved. When the two 

 eyes look at a uniformly-coloured plane surface, the retinal image is 

 precisely the same in both. But when the two eyes are directed to 

 a solid object, say a book lying on a table, the picture formed on the 

 left retina differs slightly from that formed on the right, for the left 

 eye sees more of the left side of the book, and the right eye more of 

 the right side. 



That there is a close connection between uniformity of retinal 



FIG. 306. BRRWSTER'S STEREO- 

 SCOPE. 



/ and "" are prisms, with their re- 

 fracting angles turned towards each 

 other. The prisms refract the rays 

 coming from the points c, 7 of the 

 pictures ab and u so that they 

 appear to come from a single point q, 

 Similarly, the points a and a appear 

 to be situated at /, and the points b 

 and /3 at <t>. 



