548 comparativ:b physiology. 



Estimation of the Size and Distance of Objects.— The pro- 

 cesses by which we form a judgment of the size and distance of 

 objects are closely related. 



As we have already shown (page 542), the visual angle varies 

 both with the size and the distance of an object. Knowing 

 that two objects are at the same distance from the eye, we esti- 

 mate that the one is larger than the other when the image one 

 forms on the retina is larger, or when the visual angle it sub- 

 tends is greater than in the other case, and conversely. Thus, 

 knowing that two persons are at the distance of half a mile 

 away, if one is' judged by us to be smaller than the other, it 

 will be because the retinal image corresponding to the object 

 is smaller, other things being equal. But the subject is more 

 complex than might be inferred from these statements. 



Objects of a certain color seem nearer than others ; also those 

 that are brighter, as in the case of mountains on a clear day. 

 And not only do all the qualities of the image itself enter as 

 data into the construction of the judgment, but numerous mus- 

 cular sensatiqps. The eyes accommodating and converging for 

 near objects, from the law of association, give rise to the idea of 

 nearness, for habitually such takes place when near objects are 

 viewed, so that the subject becomes very complex. That we 

 judge imperfectly of the position of an object with but one eye 

 is realized on attempting to stick a pia into a certain small spot, 

 thread a needle, cork a small bottle, etc., when one eye. is closed. 



Solidity .^ — By the use of one eye alone we can form an idea 

 of the shape of a solid body ; though, in the case of such as are 

 very complex, this process is felt to be both laborious and im- 

 perfect. 



From the limited nature of the visual field for distinct 

 vision, it follows that we can not with one eye see equally dis- 

 tinctly all the parts of a solid that is turned toward us. After 

 a little practice one may learn to define for himself what he 

 actually does see. 



Such a figure as that following results from the combina- 

 tion, mentally, of two others, which answer to the images fall- 

 ing on the right and on the left eye respectively. 



In order that such fusion shall take place, the respective 

 images must fall on identical (corresponding) parts of the 

 retina. 



As is well known, the pictures used for stereoscopes give 

 different views of the one object, as represented on a flat sur- 



