1 66 THE HUMAN BODY. 



mixed, and the brain perceives the sensation of gray or of 

 violet. Or, in other words, in order that two minute luminous 

 objects may be distinctly seen, the angle subtended upon the 

 retina by their images must not be greater than the diameter 

 of one of the retinal divisions. The distance of the two 

 objects from the eye being determined, the measure of the 

 angle subtended by them enables us to estimate the size of 

 these divisions. 



Accommodation of the eye to distances. When we make use 

 of the camera obscura, in order that the image may be dis- 

 tinct the screen must be placed in the focus of the instru- 

 ment, that is at the point where all the rays refracted by the 

 objective converge. If the objects recede or approach, the 

 screen must be placed at a proportionate distance from the 

 object-glass, so that its surface may correspond to the apices 

 of the refracted luminous cones. And yet we see with equal 

 distinctness the images of objects at very unequal distances, 

 without any variation in the form of the eye, or the relative 

 conditions of its media, or at least without our consciousness 

 of anything but a scarcely perceptible effort. This power of 

 accommodation of the eye has long been the subject of inves- 

 tigation, and the question is not yet settled. The most 

 generally received explanation is, that in order to see objects 

 at different distances, and especially very near to the eye, it 

 modifies its form, or that of its media, and adapts itself to the 

 distance in such a manner, that the retina is always in the 

 focus. According to some authors, the length of the axis of 

 the eye varies, the retina approaching or receding from the 

 crystalline. Others maintain that it is the crystalline which 

 changes its place, or that the curves of the refracting media 

 modify themselves in such a manner, as always to make the 

 apices of the luminous cones coincide with the immovable 

 retinal surface. This theory of adaptation or accommodation 

 is denied by some eminent savants, though a few of them 

 approach it in attributing this phenomenon to the contrac- 

 tion and dilatation of the pupil; while others have endeavoured 

 to demonstrate that the distance of objects from the eye may 

 vary to a great extent, without the image undergoing any 

 appreciable modification. 



