404 PHILOSOPHICAL TRANSACTIONS. [aNNO 1794. 



before the glass; the image approaching as the object recedes, and receding as 

 that approaches. For if we suppose, (fig. 2,) A and b two radiating points, from 

 which the rays ac, ad, and bc, bd, fall on the lens cd, it is manifest that 

 the rays from the nearest point a diverge more than those from the more distant 

 point B, the angle at a being greater than that of b ; consequently the rays from 

 A, whose direction is ae and af when they pass through the glass, must con- 

 vene at some point, as g, more distant from the lens than the point h, where 

 the less diverging rays bk and bl from the point b are made to convene; which 

 may also be proved by experiment with the common convex glass. It will be 

 necessary to have this proposition in view, as we shall afterwards have occasion to 

 use it in showing, that by varying the distance between the retina and the an- 

 terior part of the eye, we are enabled to see objects at different distances. 



2d. If an object, as ab, (fig. 3) be placed at a proper distance before the 

 eye, e, the rays which fall from the several points of the object falling on the 

 cornea pass through the pupil, and will be brought together by the refractive 

 power of the different parts of the eye on as many corresponding points of the 

 retina, and there paint the image of the object, in the same manner as the 

 images of objects placed before a convex lens are painted on the spectrum, 

 placed at a proper distance behind it: thus, the rays which flow from the point 

 a are united on the retina at c, and those which proceed from b are collected at 

 D, and the rays from all the intermediate points are convened at as many inter- 

 mediate points of the retina: on this union of the rays at the retina depends 

 distinct vision. But supposing the eye of a given form, should the point of 

 union lie beyond the retina, as must be the case with those from the less distant 

 object, agreeable to the preceding proposition; or should they be united before 

 they arrive at the retina, as from the more distant object, it is evident that the 

 picture at the retina must be extremely confused. Now as the rays which fall 

 on the eye from radiating points at different distances, have different degrees of 

 divergence, and the divergence of the rays increasing as the distance of the ra- 

 diating point lessens, and, vice versa, lessening as that increases; again, as those 

 rays which have greater degrees of divergence, viz. from the nearer objects, re- 

 quire a stronger refractive power to bring them together at a given distance, 

 than what is necessary to make those meet which diverge less, it is manifest, that 

 to see objects distinctly at different distances, either the refractive power of the 

 eye must be increased or diminished, or the distance between the iris and retina 

 be varied, corresponding with the different distances of the objects; both of 

 which probably take place, as will hereafter appear. 



Having then establislietl these, as our premises, we shall next examine the 

 different principles which have been employed for explaining vision at different 

 distances. 



