162 HUMAN PHYSIOLOGY 



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 x 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. 



