434 ORGANS OF THE SENSES. 



iris, like the diaphragms of optical instruments, serves in part 

 to remedy this defect. 



The chromatic aberration results from the unequal refran- 

 gibility of the various colored rays of which white light is 

 composed.: by means of this, the eye decomposes the ordinary 

 light of the objects which project it, and we see them more 

 or less colored ; the eye, in short, is not a perfect achromatic 

 organ. Habit renders us, for the most part, insensible to 

 this defect, but various experiments show it plainly. We 

 will mention only one : after looking at the cross-hairs of an 

 astronomical glass, by a red light, we shall find that, in order 

 to see them with another ray of the spectrum (another color), 

 the position of the ocular or eye-piece must be changed ; the 

 eye, when so adapted as to see by the red light, not being 

 adapted to see perfectly by the other rays of the spectrum. 



Finally, a certain irregularity of curve in the surfaces of 

 the media of the eye constitutes what is called astigmatism 

 (or monochromatic aberration). Astigmatism is such a 

 common defect in the refraction of the eye that it may be 

 said to be found, in a varying degree, in most persons ; it 

 does not, however, usually affect the sight so much as to be 

 noticed. Astigmatism consists in the more or less sensible 

 curve, from one meridian to the other of the surfaces of 

 separation between the media of the eye (especially the curve 

 of the interior surface of the cornea). Let us suppose a 

 cornea in its normal condition divided into two halves, fol- 

 lowing the line of its vertical axis, the parts maintaining their 

 original position ; the surface of the section will represent a 

 curve of a fixed radius; if the cornea be divided, following 

 its transverse axis, the surface of the section will exhibit ex- 

 actly the same curve (in a normal, non-astigmatic eye) ; in 

 other words, both sections will conform to a circumference of 

 the same radius. On the other hand, in an eye affected with 

 astigmatism (as nearly all eyes are) the radius of one will be 

 shorter than that of the other; the two curves, in short, 

 are unequal. It is easy to see that this inequality, if suffi- 

 ciently great, will interfere with the course of the luminous 

 rays as they penetrate the eye ; in fact, if we suppose that 

 the radius of one circumference is considerably shorter than 

 that of the other, we shall conclude that the eye is short- 

 sighted in the one sense, while in another it may be much 

 less so, or not at all, or even may be hypermetropic. In order 

 to remedy this defect in the refraction of the eye, it is plainly 



