188 rim.osoi'iiv or 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 sisted or superseded by those with which it is connected.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, is 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 HIRE, 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 



focus nearer the natural. In the open air, all objects, except those near, 

 were distinctly seen, hut immediately on entering a room, all was again en- 

 veloped in mist." Annuls of Philosophy, vol. x. p. 432. 



