THE EYE 161 



comparative distances of things seen. There are other 

 ways in which this is done. Even with one eye we 

 have a fair basis for drawing conclusions. It may be 

 laid down as a general principle that motor acts are 

 attended with characteristic sensations. This is true 

 of the act of accommodation. When the attention is 

 directed to a surface like the page of a book wMch is 

 only a few inches away there is a strong tension on the 

 part of the muscle that increases the curvature of the 

 lens. A feeling goes with the maintenance of this 

 tension and this feeling has, from early life, been asso- 

 ciated with looking at that which is near at hand. 

 One source of the bewilderment experienced on first 

 wearing glasses is that the accommodation effort for a 

 given distance is greater or less than before. New 

 interpretations become necessary. 



When both eyes are used there is another muscular 

 effort besides that of accommodation. This is the strain 

 of convergence, increasing as the attention is directed to 

 things close by and diminishing as we look off to the 

 distance. It will be evident that the two eyeballs must 

 be rotated considerably inward to bring the images of 

 the same word on the printed page to both the fovese 

 at once. Failure to bring the same images upon the 

 fovese results in double vision. 



So far we have mentioned the stereoscopic principle, 

 accommodation strain, and the sense of convergence as 

 contributing to our judgments of distance. Other 

 matters enter in. There is perspective, the fact made 

 familiar from experience that parallel lines retreating 

 from the eye seem to approach one another. There are 

 effects of overlapping in the landscape, more distant 

 features projecting from behind those that are not so far 

 away. There are atmospheric qualities, hazy or bluish 

 appearances which we assume to denote distance. Then, 

 too, when we know the size of an object and have an 

 unusually small image of it formed upon the retina we 

 infer that it is remote. So the apparent size of the 

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