164 HUMAN PHYSIOLOGY 



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. 

 foveae 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 

 human figure often helps us to estimate its distance from 

 us and so the distance of associated objects. These 

 devices can be used in painting upon a flat surface 

 and produce the desired effects in the absence of stereo- 

 scopic properties and differential accommodation. 



Generally speaking, we must judge distance before 

 we can judge size. It has just been said that the 

 magnitude of the image of a familiar object helps us to 

 say how far away it is but its size must be certainly 

 known if this is to be true. Bodies like rocks, which 

 may be of any size within the widest limits, cannot be 

 judged as to their actual bulk until their distance is 

 established. The discussions which people indulge in 

 regarding the size of the moon show how futile are our 

 attempts to conceive of size when there is no adequate 

 sense of the remoteness of the object under observation. 



