SENSE OF VISION. 241 



328. There are two circumstances, however, which interfere with the perfec- 

 tion of an image thus formed by a convex lens. The one is, that, if the lens 

 constitute a large part of the sphere from which it is taken, the rays which 

 fall near its margin are not brought to a focus at the same point with those 

 which pass through its centre, but at a point nearer the lens. This difference, 

 which must obviously interfere greatly with the distinctness of the image, is 

 termed spherical aberration ; it may be corrected by the combination of two 

 or more lenses, of which the curvatures are calculated to balance one another 

 in such a manner that all the rays shall be brought to the same focus ; or by 

 diminishing the aperture of the lens by means of a stop or diaphragm, in such 

 a manner that only the central part of it shall be used. The latter of these 

 methods is the one employed, where the diminution in the amount of light 

 transmitted is not attended with inconvenience! The nearer the object is to 

 the lens (and the greater, therefore, the angle of divergence of its rays), the 

 greater will be the spherical aberration, and the more must the aperture of the 

 diaphragm be contracted in order to counteract it. The other circumstance 

 that interferes with the distinctness of the image, is the unequal refrangibility 

 of the diffefently-coloured rays, which together 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 them- 

 selves 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. 



329. 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 degree 

 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 ordinarily read), the 

 f rays proceeding from an object, placed nearer to the eye than this, would not 

 be brought to a focus upon the retina, but Avould converge towards a point behind 

 it; whilst, on the contrary, the rays from an object at a greater distance would 

 meet before they reached the retina, and would have again diverged from each 

 other when they impinge upon it; so that, in either case, vision would be indis- 

 tinct. Now two methods of adaptation suggest themselves to the Optician. 

 Either he may vary the distance between the refracting surface and the screen 

 on which the image is formed, in such a manner, that the latter shall always 

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