244 FERGUSON'S LECTURES. 



blinded ; but it is so soon over, that we are scarce 

 v~x-v-^/ ever sensible of it. 



why some Some eyes require the assistance of convex glasses to 

 quire spec- make them see objects distinctly, and others of concave, 

 tacles. if either the cornea a b c or chrystalline humour e, or 



both of them, be too flat, as in the eye A, their focus 

 will not be on the retina, as at d, where it ought to be 

 in order to render vision distinct ; but beyond the eye, 

 as at f. Consequently those rays which flow from the 

 object C, and pass through the humours of the eye, are 

 not converged enough to unite at d ; and therefore the 

 observer can have but a very indistinct view of the ob- 

 ject. This is remedied by placing a convex glass g h 

 before the eye, which makes the rays converge sooner, 

 and imprints the image duly on the retina at d. 



If either the cornea, or chrystalliue humour, or both of 

 them, be too convex, as in the eye^, the rays that enter 

 in from the object C, will be converged to a focus in 

 the vitreous humour, as at f; and by diverging from 

 thence to the retina, will form a very confused image 

 thereon : and so, of course, the observer will have as 

 confused a view of the object, as if his eye had been 

 too flat. This inconvenience is remedied by placing a 

 concave glass g h before the eye ; which glass, by caus- 

 ing the rays to diverge between it and the eye, lengthens 

 the focal distance so, that if the glass be properly cho- 

 sen, the rays will unite at the retina, and form a distinct 

 picture of the object upon it. 



Such eyes as have their humours of a due convexity 

 cannot see any object distinctly at a less distance than 

 mx inches ; and there are numberless objects too small 

 to oe seen at that distance, because they cannot appear 



