422 THE SENSORIAL FUNCTIONS. 



one distance, it will have confused images of objects 

 at another distance ; because the exact foci of the 

 rays will be situated either before or behind the 

 retina. It is evident that if the retina be not placed 

 exactly at the point where the focus is situated, it 

 will either intercept the pencil of rays before they 

 are united into a point, or receive them after they 

 have crossed one another in passing through the 

 focus ; in either of which cases, each pencil will 

 throw upon the retina a small circle of light, 

 brighter at the middle and fainter at the edges, 

 which will mix itself with the adjacent pencils, 

 and create confusion in the image. 



It is found, however, that the eye has a power 

 of accommodating itself to the distinct vision of 

 objects at a great variety of distances, according 

 as the attention of the mind is directed to the par- 

 ticular object to be viewed. That the refractive 

 powers of the eye really change, is easily proved 

 by the experiment of admitting only two separate 

 pencils of rays proceeding from any small object, 

 such as the head of a pin, to fall upon the eye ; as 

 we can then at once ascertain whether or not they 

 meet at the retina ; for when they do so, they form 

 one image only, and the object appears single ; 

 whereas if they do not unite, two images are formed 

 on the retina, giving rise to the perception of a 

 double object. This separation of the pencils may 

 be obtained by viewing the object through two 

 pin holes, made nearer to one another than the 

 diameter of the pupil, in a blackened card held 

 close to the eye. It will then be found that, with- 

 out altering the situation either of the eye, or the 

 object, or the interposed card, we possess the power, 



