OBSERVATIONS ON SOUNDNESS/^ 
By R. H. Dyer, M.R.C.V.S., Waterford. 
(Continuedfromp. 340.) 
^^Our idea of the distance of near objects is evidently ac¬ 
quired from experience, and is suggested by the muscular 
, sensations which are produced by the contraction of the 
adductor muscles of the eyes. When we direct our eyes 
towards a near object, a certain degree of convergence takes 
place between their axes; the degree increasing as the dis¬ 
tance between the object and the eyes diminishes, and vice 
versa. We instinctively interpret the sensations thus pro¬ 
duced, in such a manner as to be able to compare, with great 
accuracy, the relative distances of two objects that are not 
remote from the eyes. This intuition, however, is evidently 
one of the acquired kind, as may be seen by watching the 
actions of an infant, or of a person who has recently become 
possessed of vision. When an object is held before the eyes, 
and an attempt made to grasp it, the manner in which the 
attempt is made clearly shows that there is no power of 
forming a precise idea of its situation, such as that which 
exists in many of the lower animals from their first entrance 
into the world. 
The impressions made upon the eyes have to be corrected 
by those received through the touch before the power of judg¬ 
ing of distance is acquired. How much this power depends 
upon the conjoint use of hotli eyes, is evident from the diffi¬ 
culty with which any actions that require an exact appreciation 
of distance are performed by those who have lost the sight of 
one eye until they have acquired new modes of judging of it. 
In regard to remote objects we have not the same guide, 
since the convergence of the eyes, in viewing them, is so slight 
that the axes are virtually parallel. Our judgment of their dis¬ 
tance is chiefly founded upon their apparent size, if their actual 
size be knowm 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 form our judgment 
chiefly from the greater or less distinctness of their colour 
and outline. Hence our idea of it will be very much affected 
by varying states of the atmosphere, a slight haziness in¬ 
creasing the apparent distance, whilst a peculiarly clear state 
of the air will seem to cause remote objects to approach 
