ESTIMATION OF DISTANCE AND SIZE. 
437 
remote ; or whether, being in a contemplative mood, his eyes 
are fixed npon 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, 
