jgg Dr. Hosack's Observations on Vision. 
object 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 rad’ating points, from which the rays 
A C, A D, and B C, B D, fall upon the lens C D, it is manifest 
that the rays from the nearest point A diverge more t.ian 
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 A E and A F when they pass through the glass, 
must convene at some point (as G) more distant from the 
lens than the point H, where the less diverging rays B K and 
B L from the point B are made to convene ; which may also 
be proved by experiment with the common convex glass.f 
It will be necessary to have this proposition in view, as we 
shall afterwards have occasion to use it in shewing, that by 
varying the distance between the retina and the anterior part 
of the eye we are enabled to see objects at different distances. 
ad. If an object, as A B, (fig. 3.) be placed at a proper dis- 
tance before the eye (E), the rays which fall from the several 
points of the object falling upon 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 upon 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 intermediate points of the 
* Euclid Book I Prop. 21. 
f See Kepler. Diopt. Postul. Smith’s Optics, Gravesande, &c. 
