ADAPTATION OF THE EYE TO DISTANCES. 265 



refractive powers, by an alteration of its shape or density ; or, Thirdly. 

 In viewing near objects, those rays only must be admitted, which are 

 nearest the axis of the eye, and are consequently least diverging. 



1. The hypothesis, that the adjustment of the eye is dependent upon 

 an alteration of the antero-posterior diameter of the organ, or on the 

 relative position of the humours and retina, has been strongly supported 

 by many able physiologists. Blumenbach 1 was of opinion, and his views 

 seem to have been embraced by Dr. Hosack, 2 that the four straight 

 muscles of the eye, by compressing the eyeball, cause a protrusion of 

 the cornea, and thus increase the length of the axis. Dr. Monro se- 

 cundus 3 believed, that the iris, recti muscles, the two oblique, and the 

 orbicularis palpebrarum participate in the accommodation; and Ham- 

 berger, Briggs, 4 and others, that the oblique muscles, being thrown in 

 opposite directions around it, may have the effect of elongating the 

 axis of the eye. Kepler 5 thought, that the ciliary processes draw the 

 crystalline forward, and increase its distance from the retina. Des 

 Cartes 6 imagined the same contraction and elongation to be effected by 

 muscularity of the crystalline, of which he supposed the ciliary pro- 

 cesses to be the tendons. Porterfield, 7 that the corpus ciliare is con- 

 tractile, and capable of producing the same effect. Jacobson, 8 that the 

 aqueous humour, by entering the canal of Petit through the apertures 

 in it, distends the canal, and pushes the crystalline forward. Sir Eve- 

 rard Home, 9 that the muscular fibres, which he has described as exist- 

 ing between the ciliary processes, move the lens nearer to the retina, 

 and that the lens is brought forward by other means, (which he leaves 

 to conjecture,) when the distance of the object is such as to require it. 

 Dr. Knox, 10 that the annulus albus, or the part which unites the choroid 

 and sclerotic coats, is muscular, and the chief agent in this adjustment. 

 Professor Mile, 11 of Warsaw, that the contraction of the iris changes 

 the curvature of the cornea; whilst Sir David Brewster 12 thinks it 

 "almost certain, that the lens is removed from the retina by the con- 

 traction of the pupil." 



Without examining these and other views in detail, it may be re- 

 marked, that the nicest and most ingenious examination by the late 

 Dr. Young 13 could not detect any change in the length of the axis of 

 the eyeball. To determine this, he fixed his eye, and at the same time 

 forced in upon the ball the ring of a key, so as to cause a very accu- 

 rately defined phantom within the field of perfect vision; then looking 



1 Instit. Physiolog., 276, or Elliotson's translation. 



2 Philosoph. Transact, for 1794, p. 146. 



3 Three Treatises on the Brain, the Eye, the Ear, p. 137, Edinb., 1797. 



4 Nova Visionis Theoria, Lond., 1685. & Haller, Element. Physiol., xvi. 4, 2. 



6 De Homine, p. 45, Lugd. Bat, 1664. 



7 A Treatise on the Eye, the Manner and Plienomena of Vision, Edinb., 1759. 



8 Magendie, Precis, &c., i. 78. 



9 Philosoph. Transact, for 1794, 1795, 1796, and 1797; and Lectures on Comparative 

 Anatomy, iii. 213, Lond., 1823. 



10 Edinb. Philos. Transact., x. 56. 



Magendie, Journal de Physiologic, vi. 166; and Elliotson's Human Physiology, p. 571, 

 Lond., 1840. 



12 Edinburgh Journal of Science, i. 77; and Treatise on Optics, op. chat, p. 253. 



13 Philos. Transact, for 1795. 



