ESTIMATION OP DISTANCE AND SIZE. 437 



remote ; or whether, being in a contemplative mood, his eyes 

 are fixed upon no definite object, but are looking into space. 

 In the latter case, as in the case of blind persons in whose 

 eyes there is no other indication of loss of sight, the peculiar 

 vacant expression is due to the want of any convergence 

 between the axes of the eyes, such as would indicate that 

 they are fixed upon an object. The assistance which the 

 joint use of both eyes affords in the estimation of distance, is 

 evident from the fact, that, if we close one eye, we are unable 

 to execute with certainty many actions which require a 

 precise appreciation of the distance of near objects, such as 

 threading a needle, or snuffing a candle. Instances are not 

 unfrequent in which persons have first become aware, by 

 experiencing this difficulty, that they had lost the sight of' 

 one of their eyes. 



565. In regard to distant objects, our judgment is chiefly 

 founded upon their apparent size, if their actual size be 

 known to us, and also upon the extent of ground which we 

 see to intervene between ourselves and the object. But if 

 we do not know their actual size, and are so situated that we 

 cannot estimate the intervening space, we principally form 

 our judgment from the greater or less distinctness of their 

 colour and outline. Hence, this estimate is liable to be very 

 much affected by varying states of the atmosphere ; a distant 

 ridge of hills, for example, sometimes appearing to be more 

 remote, at other times to be comparatively near, according as 

 the air is hazy or peculiarly clear. 



566. Our notion of the size of an object is closely con- 

 nected with that of its distance. It is founded on the 

 dimensions of the picture formed by the object upon the 

 retina ; but it is corrected by the known or supposed distance 

 of the object itself. Thus, I hold up a book at a certain 

 distance from my eye, and it covers the whole of the opposite 

 window ; the apparent size of both pictures, therefore, is just 

 the same ; but knowing that the book is much nearer than 

 the window, I infer that it is much smaller. When we know 

 their respective distances, the estimate of their real sizes is 

 very easy : but this is not the case when we only guess-at 

 their distances. Hence our estimation of the size of objects 

 even moderately distant, is much influenced by states of the 

 atmosphere. Thus, if we walk across a common in a fog, 



