188 PHILOSOPHY OF ZOOLOGY. 
When it is considered that the supporters of these vari- 
ous opinions have discovered in each of the parts of:the eye 
which have been referred to, a provision for enabling it to 
see distant and near objects, it seems not unreasonable to 
conclude, that the same part may not exercise the same 
function in all animals, but, in the different classes, be as- 
sisted or superseded by those with which it is connected. 
It is likewise probable, that the necessity of this power of 
adjustment may not exist to the extent which has been sup- 
posed, and that the limits of distinct vision are included 
within a narrow range. ‘The human eye sees objects most 
distinctly at the distance of from six to ten inches. When 
these are removed to a greater distance, we do not perceive 
so clearly the shades of colour, or the inequalities of the sur- 
face, and this indistinctness of vision increases with the dis- 
tance. The action of the straight muscles, however, serves 
in some degree to correct the defect. But, in looking at 
distant objects, we are assisted greatly in our perceptions 
by our former experience; so that it may often happen 
that the praise which we bestow on the sightfulness of the 
eye, 1s due to the readiness of the recollection. Microsco- 
pical inquiries are seldom prosecuted so habitually as to 
furnish the same aid to the unassisted eye, when viewing 
objects within the range of minute vision. 
Some physiologists have been disposed to conclude, that 
the formation of a perfect image on the retina, is not essen- 
tial to distinct vision; and the following experiment of M. 
DE LA Hrrg, has been brought forward in support of the 
opinion : “ If a small object placed at that distance from 
the eye, at which vision is most distinct, be viewed through 

Yocus nearer the natural. In the open air, all objects, €xcept those near, 
were distinctly seen, but immediately on entering a room, all was again en- 
weloped in mist.”” Annals of Philosophy, vol. x. p. 432. 
