64 On the Adjustment of the Eye to see 



pose the point F to be given, or the length of the axis B F 

 to be imiputable, and tnen the quantity of the refraction of 

 each ray must be varied. And because the distance CD 

 is supposed equal to C B, or C A, the angle C A D is equal 

 to C D A, and consequently to DAE. Therefore, con- 

 ceiving each ray to come back again from the fixed point 

 F, to the points C, D, E, successively ; the whole quantity 

 of its refractions must first be lessened by the angle CAD, 

 and then by the equal angle DAE; and so the changes of 

 the figures of the refracting surfaces must be much the 

 same when the object is removed from C to D, as when it 

 is removed from D to E*." 



Hence it may be demonstrated, that objects at various 

 distances, as C, D, and E, may be seen distinctly with- 

 out any alteration in the humours of the eye, or in its out- 

 ward form, 



1. It has been proved in a former paper f, that when a 

 good eye views an object at the least distance it can be seen 

 distinctly, the rays w^hich enter the pupil are parallel, or 

 such as differ very little from being so; whence it is evi- 

 dent, that the rays from the object C falling upon the eye 

 at B are parallel. 



2. Rays issuing from the object E, a planet or a star, at 

 an infinite distance are parallel, consequently the angle 

 DA E is infinitely small: and 



3. As the angle CD A is equal to the angle DAE, the 

 angle CD A is infinitely small ; therefore the line DA co- 

 incides with the line DB, and the rays falling upon the 

 eye from D must also be parallel ; consequently the rayt 

 from the object D, and also those from E and C, are re- 

 fracted to the same point F upon the retina : for parallel 

 rays falling upon the eye near the axis of vision have the 

 same focus, whether they come from objects that are near 

 or remote. Whence it is evident that vision perfectly di- 

 stinct is produced only by parallel rays ; but the means by 

 which the eye admits such rays only as are parallel, or 

 nearly so, and rejects the rest, come next to be considered. 



III. — If a small circular object be viewed with too large 

 a pupil, which may be done by placing the object much 

 within the limits of distinct vision, it will appear larger 

 than perfect vision would represent it, by a penumbra of 

 light, which is called the circle of dissipation. 



Now if the pupil be contracted, by means of a perfora- 



* Smith's Opticf, Remarks, p. 2. 



I Philosophical Magazine, vol, xxix. p. 342. 



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