Oljects distinctly at different Distances, 85 



tion made in a card of such magnitude as to prevent the 

 lateral rays from entering the eye, the object will appear 

 distinct. But if the aperture in the card be made as large as 

 the pupil, the circle of dissipation will appear as large as 

 before. 



This circle of dissipation is formed by those rays which 

 enter the eye remote from the axis of the crystalHne lens. 

 Thus, when the pupil is too large for distinct vision, the 

 most refrangible of those side rays will cross one another in 

 the vitreous humour, and, by falling upon the retina in a 

 diverging state, will be disperses! over a larger space than 

 the true image, and consequently form a penumbra round 

 it ; and the least refrangible rays of the same pencil will be 

 dispersed over the interior parts of the circle ; whence that 

 indistinctness of vision which is experienced by people ad- 

 vanced in years. 



No writer has paid more attention to the theory of distinct 

 and indistinct vision than Dr. Jurin. This philosopher says, 

 that '' the radius of dissipation is, cofteris paribus, always 

 proportional to the radius of the pupil. Consequently, 

 when the pupil is narrow, the radius of dissipation and the 

 penumbra arising from the dissipation will be smaller, that 

 IS, vision will be rendered either distinct, or at least less 

 indistinct than it would otherwise be*." 



IV. — Many philosophers have maintained that we have 

 the power of viewing objects at different distances, by a 

 conformation of the eye for this purpose, independent of a 

 variation in the pupil; but they vary much in their opi- 

 nions respecting the means by which this effect is pro- 

 duced. 



Dr. Matthew Young says, that " the power of seeing 

 distinctly at diff*erent distances <l(oes iK)t depend on the cry- 

 stalline. 



*' This is evident," he says, " from the experiments 

 made on a person who had been couched for a cataract, and 

 by the assistance of the same convex lens, applied to that 

 eye, could see distinctly at different distances f." 



But Dr. Porterfield is of opinion, that the change made 

 in the eye must be in the crystalline; for a ptrson who ha^* 

 been couched of a cataract was under the necessity of using 

 glasses of different degrees of convexity, for seeing objects 

 distinctly at different distances:}:. 



These two allegt^d facts may both be true, but the con- 



* Jurin'a Essay on distinct and indistinct Vision, p, 145. 

 f Dr. M. Young's Analysis of Nat. Pkil. p. 37.5. 

 i See Porterfi^UTon the Eye, vol. i. p. 434, 435. 



F 3 elusion 



