186 Experimental Philosophy. [Lecture 13. 



the eye, you will also perceive the causes of 

 distinct and indistinct vision. To see an object 

 distinctly, it is necessary that every pencil of 

 diverging rays, which reaches the eye from the 

 object, should be converged to a point on the 

 optic nerve, corresponding to that from which 

 the rays have diverged. If, on the contrary they 

 are brought in an unconverged state to the retina, 

 you may easily conceive that the particles of light 

 will be so scattered and dispersed, as to make an 

 indistinct impression. This last defect takes 

 place when the eye, by age or infirmity, is made 

 flat, and consequently is not sufficiently convex to 

 cause the rays to converge in their proper place ; 

 persons with this defect can often see objects 

 better at a great distance than very near. The 

 opposite fault to this is when the eye is too convex, 

 when the rays will be made to unite too soon, 

 before they reach the retina ; persons with this 

 defect, therefore, are called short sighted because 

 they can only discern objects which are very near 

 to the eye. 



I have seen a very pretty contrivance in the 

 shop of an optician, illustrative of the causes of 

 weak and short sight. Two eyes were made of 

 glass, as fig. 68 and 69, and the pencils of diverg- 

 ing rays, issuing from three points, were repre- 

 sented by threads of silk of three different colours. 

 Thus in fig. 68, which represents weak or in- 

 distinct vision, you see the rays are not united 

 in points when they reach the back of the eye, 



