ADJUSTMENT OF EYE TO DISTANCE. 439 



different foci, the retina does not receive a distinct impression of 

 either ; but circles of dissipation, as they are termed, form around the 

 images of the objects. If, however, the eye be directed first to one, 

 and then to the other object, they are both distinctly perceived in suc- 

 cession. In such acts, one is conscious of certain change and effort 

 within the eye. When the eyes are directed from a distant to a near 

 object, the change is apparently due to some internal muscular con- 

 traction, which ceases the moment distant objects are looked at, the 

 parts then resuming their natural and unconstrained position. The 

 sense of fatigue, which always attends near vision, if long continued, 

 is immediately relieved when the eyes are directed to distant objects, 

 the state most commonly regarded as that proper to the eye when at 

 rest. 



Concerning the nature of the changes which the eye undergoes, in its 

 adjustment for different distances, various theories have been advanced. 

 According to one view, the adaptation of the eye to distance is effected 

 by means of changes in the condition of the iris, inasmuch as in near 

 vision the pupil is contracted, and in distant vision dilated. The con- 

 traction of the pupil, which always takes place when near objects are 

 viewed, may, by excluding the marginal rays, help to prevent the for- 

 mation of circles of dissipation, and thus to render the images of ob- 

 jects more distinct. On this principle a small object held close to the 

 eye, and therefore seen indistinctly, may be rendered distinct by look- 

 ing at it through a small pin-hole in a card; this cuts off the marginal 

 rays; at the same time, the object appears less bright, and also mag- 

 nified, because its image on the retina is larger, owing to its proximity 

 to the eye. The contraction of the pupil is not, however, the sole 

 change that takes place in near vision, and certainly not the efficient 

 change; for on looking at a bright luminous body situated at a great 

 distance, the pupil contracts, whereas if the eye be directed to a near 

 object of feeble luminous power, the pupil dilates. Moreover, the ad- 

 justment of the eye can be effected, even when looking through an 

 unchangeable pin-hole in a piece of paper; further, in cases in which 

 the iris is wanting, or in which it has been entirely removed, the power 

 of adaptation of the eye has remained perfect; and lastly, in long-sighted 

 persons, although the pupil may contract with great vigor, yet near 

 objects are very indistinctly seen. 



The movements of the iris under the several above-mentioned con- 

 ditions, seem to be consentaneous, being primarily regulated according 

 to the quantity of light entering the eye, which is relatively more 

 intense, from near objects. 



The adaptation of the eye to vision at different distances has been 

 referred, by some, to alterations in the distance between the optic 

 centre of the eye and the retina, produced by the pressure of the ocu- 

 lar muscles upon the eyeball. Thus it was maintained that, in near 

 vision, the antero-posterior diameter of the eyeball is lengthened by 

 the action of these muscles, according to some by the recti, according 

 to others by the oblique muscles. But to be efficient in accommodating 

 the eye to the necessary distances, such elongation or shortening of 

 the eyeball must be greater than can possibly occur in it. Moreover, 



