VOL. LXXXIV.] VHILOSOPHICAL TRANSACTIONS. 403 



XF. Observations on Vision. By David Hosach, M. D. p. ] 96. 



By what power is the eye enabled to view objects distinctly at different distan- 

 ces? As the pupil is enlarged or diminished according to the greater or less quan- 

 tity of light, and in a certain degree to the distance of the object, it would 

 readily occur that these different changes of the pupil would account for the 

 phenomena in question. Accordingly anatomists and philosophers, who have 

 written on this subject, have generally had recourse to this explanation. 



Amusing myself with these changes of ihe pupil, as a matter of curiosity, 

 says Dr. H., by presenting to the eye different objects at different distances, I 

 soon perceived that its contraction and dilatation were irregular and more limited 

 than had been supposed; i. e. that approaching the object nearer the eye, within 

 a certain distance, the pupil not only ceased to contract, but became again di- 

 lated; and that beyond a few yards distance, it also ceased to dilate: these cir- 

 cumstances immediately occurred as objections to the above explanation; for 

 were it from the contraction and dilation of the iris alone that we see objects at 

 different distances, I naturally concluded it should operate regularly to produce 

 its effects; but if to view an object at a few yards distance it be enlarged to the 

 utmost 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 proposed 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. Encouraged by their replies, I have since at- 

 tended more particularly to the subject, and hope my inquiries have not been al- 

 together 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 re- 

 ference. 



1st. Let ABC, pi. 5, fig. 1, be an object placed before the double convex 

 lens DE, at any distance greater than the radius of the sphere of which the lens 

 is a segment: the rays which issue from the different points of the object, and 

 fall on 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 object 



3 F 2 



