•260 optics. 



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 object. This is remedied by 

 placing a concave glass, g h, of a proper focus, 

 before the eye; which makes the rays converge 

 sooner, and imprints the image duly on the retina 

 at d. 



If either the cornea or crystalline humour, or 

 both of them, be too convex, the rays that enter in 

 from the object C (Fig. 4.) will be converged to a 

 focus in the vitreous humour, as at f, and by di- 

 verging 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 inconveni- 

 ence is remedied by placing a concave glass, g h, 

 before the eye ; which glass, by causing 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. 



When glasses are put in frames for spectacles, 

 their frames ought not to be straight, so as both 

 eyes may be as the same plane ; but they ought to 

 be so bent in the middle, that the axis of both 

 glasses may be directed to one point, at such a dis- 

 tance as you generally look with spectacles. By 

 this means the eyes will fall perpendicular upon 

 both glasses, and make the object appear distinct. 

 But if they fall obliquely upon the glasses, it will 

 cause a confused appearance in the objects; there- 

 fore the shape of the frame ought to be as repre- 

 sented (Fig. 7.) when A B is the plane as one glass 

 is fixed, and C D the other. 



