874 OF SENSATION, AND THE ORGANS OF THE SENSES. 



consequently greater. The rays diverging from the several points of a luminous 

 object are thus brought to a corresponding focus ; and the places of all these 

 foci hold exactly the same relation to each other, with that of the points from 

 which the rays diverged; so that a perfect image of the object is formed 

 upon a screen held in the focus of the lens. This image, however, will be in- 

 verted ; and its size, in proportion to that of the object, will depend upon their 

 respective distances from the lens. If their distances be the same, their size 

 will also be the same; if the object be distant, and the image near, the latter 

 will be much the smaller ; and vice versa. 



876. 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 con- 

 tracted in order to counteract it. The other circumstance that interferes with 

 the distinctness of the image is the unequal refrangibility of the differently 

 colored rays, which together make up white or colorless 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 colored rays) differs con- 

 siderably. This is the case with flint and crown glass for instance the dis- 

 persive 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 dis- 

 persion of the rays effected by the former will be entirely counteracted by the 

 latter, which diminishes in part only its refractive power. 



877. The Eye may be regarded as an optical instrument of great perfection, 

 adapted to produce, on the expanded surface of the Optic nerve, a complete 

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

 colors, lights and shades, &c. of the object are all accurately represented. 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 colors. The power by which it adapts itself to varia- 

 tions 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 rays proceeding from an 



