243 



By the point of distinct vision, is understood the distance at which we 

 can read a book of which the characters are of middling size, or distin- 

 guish any other object equally small. This distance is not confined within 

 very narrow limits, since we can read the same book at six inches from 

 the eye, or at five or six times the distance. This faculty of the eyes, to 

 adapt themselves to the distance and the smallness of objects, cannot 

 depend, as has been often repeated, on the lengthening or shortening 

 of the globe of the eye by the muscles that move it. Its four recti 

 muscles are not, in any case, capable of compressing it on its sides, 

 nor of lengthening it by altering its spherical form; their simultaneous 

 action can only sink the ball in its socket, flatten it from the fore to the 

 back part, diminish its depth, and make the refraction, consequently, 

 less powerful, when objects are very distant or very small: this last effect, 

 even, might be disputed. The eye which moves and rests on the adipose 

 cushion which fills the bottom of the socket, is never strongly enough 

 pressed to lose its spherical figure, which of all the forms in which bo- 

 dies can be invested, is that which, by its especial nature, best resists 

 alteration. The extremities of the ciliary processes, which surround the 

 circumference of the crystalline lens, cannot act on this transparent lens, 

 compress nor move it: for, these little membranous folds, of which the 

 aggregate composes the irradiated disk, known under the name of corpus 

 ciliare, possessing no sort of contractile power, are incapable of moving 

 the crystalline lens with which their extremities lying in simple conti- 

 guity have no adherence, and which, besides, is immovably fixed in the 

 depression which it occupies, by the adhesions of its capsule with the 

 membrane of the vitreous humour. The various degrees of contraction 

 or dilation of which the eye-ball is susceptible, afford a much more satis- 

 factory explanation of this physiological problem. 



The rays of light which come from a very near object, are very diver- 

 gent: the eye would want the refracting power necessary to collect them 

 into one, if the pupil, contracting by the enlargement of the iris, did 

 not throw off the more divergent rays, or those which form the circum- 

 ference of the luminous cone. Then, those, which form the centre of 

 the cone, and which need but a much smaller refraction for their re-union 

 on a single point of the retina, are alone admitted by the straightened 

 opening. When, on the contrary, we look at a distant object from 

 which the rays are given out, already very convergent, and which need 

 but a small refraction to bring them towards the perpendicular, we di- 

 late the pupil, in order to admit the more diverging rays, which, when 

 collected, will give the image of the object. In this respect, very small 

 bodies are on the same footing as those at a great distance. 



Though the image of every object is traced at the same time in both our 

 eyes, we have but one sensation, because the two sensations are in harmony 

 and are blended, and serve only, one aiding the other, to make the impres- 

 sion stronger and more durable. It has long been observed, that sight is 

 more precise and correct when we use only one eye, and Jurine thinks 

 that the power of the two eyes united exceeds only by one thirteenth, 

 that of a single eye. The correspondence of affectton requires the di- 

 rection of the optical axes on the same objects, and be that direction 

 ever so little disturbed, we see really double, which is what happens in 

 squinting. 



If the eyes are too powerfully refractive, either by the too great con- 



