THE SENSES. 213 



those traversing its central portion. This defect is known as spherical 

 aberration, and in the camera, telescope, microscope, and other optical in- 

 struments, it is remedied by the interposition of a screen with a circular 

 aperture in the path of the rays of light, cutting off all the marginal rays 

 and only allowing the passage of those near the centre. Such correction is 

 effected in the eye by the iris, which forms an annular diaphragm to cover 

 the circumference of the lens, and to prevent the rays from passing through 

 any part of the lens but its centre which corresponds to the pupil. The 

 posterior surface of the iris is coated with pigment, to prevent the passage 

 of rays of light through its substance. The image of an object will be 

 most denned and distinct when the pupil is narrow, the object at the 

 proper distance for vision, and the light abundant; so that, while a suffi- 

 cient number of rays are admitted, the narrowness of the pupil may pre- 

 vent the production of indistinctness of the image by spherical aberra- 

 tion. But even the image formed by the rays passing through the cir- 

 cumference of the lens, when the pupil is much dilated, as in the dark, 

 or in a feeble light, may, under certain circumstances, be well denned. 



Distinctness of vision is further secured by the outer surface of the 

 retina as well as the posterior surface of the iris and the ciliary processes, 

 being coated with black pigment, which absorbs any rays of light that 

 may be reflected within the eye, and prevents their being thrown again 

 upon the retina so as to interfere with the images there formed. The 

 pigment of the retina is especially important in this respect; for with the 

 exception of its outer layer the retina is very transparent, and if the sur- 

 face behind it were not of a dark color, but capable of reflecting the 

 light, the luminous rays which had already acted on the retina would be 

 reflected again through it, and would fall upon other parts of the same 

 membrane, producing both dazzling from excessive light, and indistinct- 

 ness of the images. 



5. Chromatic Aberration. In the passage of light through an 

 ordinary convex lens, decomposition of each ray into its elementary 

 colored parts commonly ensues, and a colored margin appears around the 

 image, owing to the unequal refraction which the elementary colors un- 

 dergo. In optical instruments this, which is termed chromatic aberration, 

 is corrected by the use of two or more lenses, differing in shape and density, 

 the second of which continues or increases the refraction of the rays pro- 

 duced by the first, but by recombining the individual parts of each ray 

 into its original white light, corrects any chromatic aberration which may 

 have resulted from the first. It is probable that the unequal refractive 

 power of the transparent media in front of the retina may be the means 

 by which the eye is enabled to guard against the effect of chromatic aber- 

 ration. The human eye is achromatic, however, only so long as the 

 image is received at its focal distance upon the retina, or* so long as the 

 eye adapts itself to the different distances of sight. If either of these 



