Dr . Hosack's Observations on Vision. 
1 97 
extent, surely it must of itself be insufficient to view one at 
the distance of several miles ; for example, the heavenly 
bodies. 
Another difficulty here presents itself : in viewing the sun, 
instead of dilating, according to the distance, it contracts, 
obeying rather the quantity or intensity of the light, than the 
distance of the object. Knowing no other obvious power in 
the eye itself of adapting it to the different distances of objects, 
it occurred to me to inquire, whether the combined action of 
the external muscles could not have this effect. I first pro- 
posed this query to an optician of eminence in London, and 
who has written expressly on this subject. I repeated the 
same question to a celebrated teacher of anatomy. Encou- 
raged by their replies, I have since attended more particularly 
to the subject, and hope my inquiries have not been altogether 
unsuccessful. As introductory to a more distinct view of 
what I have to advance, it appears necessary to premise the 
following observations, relative to those general laws of vision 
which are more particularly connected with this part of the 
subject, and to which we shall have occasion of frequent 
reference. 
ist. Let A B C, (Tab. XVII. fig. 1.) be an object placed before 
the double convex lens D E, at any distance greater than the 
radius of the sphere whereof the lens is a segment ; the rays 
which issue from the different points of the object, and fall 
upon the lens, will be so bent by the refractive power of the 
glass as to be made to convene at as many other points behind 
the lens, and at the place of their concourse they will form 
an image .or picture of the object. The distance of the image 
behind the glass varies in proportion to the distance of the 
