408 ON SENSATION, AND THE ORGANS OF THE SENSES. 



make up white or colourless light ; the violet being more bent from their 

 course than the blue, the blue more than the yellow, and the yellow more 

 than the red ; the consequence of which will be, that the violet rays are 

 brought to a focus much nearer to the lens than the blue, and the blue nearer 

 than the red. If a screen be held to receive the image, in the focus of any 

 of the rays, the others will make themselves apparent as fringes round its 

 margin. This difference is termed Chromatic Aberration. It is corrected in 

 practice, by combining together lenses of different substances, of which the 

 dispersive power (that is, the power of separating the coloured rays) differs 

 considerably. This is the case with flint and crown glass, for instance, the 

 dispersive power of the former being much greater than that of the latter, 

 whilst its refractive power is nearly the same : so that, if a convex lens of 

 crown glass be united with a concave of flint whose curvature is much less, 

 the dispersion of the rays effected by the former will be counteracted by the 

 latter, which diminishes in part only its refractive power. 



536. The Eye may be regarded as an optical instrument of great perfec- 

 tion, adapted to produce, on the expanded surface of the optic nerve, a com- 

 plete image or picture of luminous objects brought before it; in which the 

 forms, colours, lights and shades, &c.., of the object are all accurately repre- 

 sented. By the different refractive powers of the transparent media, through 

 which the rays of light pass, and by the curvatures given to their respective 

 surfaces, both the Spherical and Chromatic aberrations are corrected in a de- 

 gree sufficient for all practical purposes : so that, in a well-formed eye, the 

 picture is quite free from haziness, and from false colours. The power by 

 which it adapts itself to variations in the distance of the object, so as to 

 form a distinct image of it, whether it be six inches, six yards, or six miles 

 off, is extremely remarkable, and cannot be regarded as hitherto completely 

 explained. It is obvious that, if we fix upon any distance as that for which 

 the eye is naturally adjusted (say 12 or 14 inches, the distance at which we 



(Fig. 173. 



' A longitudinal section of the globe of the Eye; 1, the sclerotic, thicker behind than in front; 8, the 

 cornea, received within the anterior margin of the sclerotic, and connected with it by means of a be- 

 veled edge; 3, the choroid, connected anteriorly with (4) the ciliary ligament, and (5) the ciliary pro- 

 cesses ; 6, the iris ; 7, the pupil ; S, the third layer of the eye, the retina, terminating anteriorly by an 

 abrupt border at the commencement of the ciliary processes ; 9, the canal of Petit, which encircles the 

 lens (12) ; the thin layer in front of this canal is the /onula ciliaris, a prolongation of the vascular layer 

 of the retina to the lens. 10, the anterior chamber of the eye, containing the aqueous humour; the 

 lining membrane by which the humour is secreted is represented in the diagram ; 11, the posterior; 12, 

 tlit lens more convex behind than before, and enclosed in its proper capsule; 13, the vitreous humour 

 enclosed in the hyaloid membrane, ami in cells formed in its interior by that membrane ; 14, a tubular 

 sheath of the hyaloid membrane, which serves for the passage of the artery of the capsule of the lens ; 

 15, the neurilemma of the optic nerve ; 1C, the arteria centralis retinae, imbedded in its centre.] 



