THE SENSE OE VISION. 



317 



On the retina itself will be formed not an accurate optical image of the point 

 A, but a small circle of dispersion in which the various colored rays are mixed 

 together, the violet rays after crossing falling upon the same part of the retina 

 as the red rays before crossing. Thus by a sort of compensation, which, how- 

 ever, cannot be equivalent to the synthetic reproduction of white light by the 

 union of the spectral colors, the disturbing effect of chromatic aberration is 

 diminished. When the lower half of the pupil is covered by the edge of a 

 card held in front of the cornea at D, the aberration produced in the upper 

 half of the eye is not compensated by that of the lower half. Hence the 

 image of a point of white light at A will appear as a row of spectral colors 

 on the retina, and all objects will appear bordered by colored fringes. Another 

 good illustration of the chromatic aberration of the eye is obtained by cutting 

 two holes of any convenient shape in a piece of black cardboard and placing 

 behind one of them a piece of blue and behind the other a piece of red glass. 

 If the card is placed in a window some distance (10 meters) from the observer, 

 in such a position that the white light of the sky may be seen through the col- 

 ored glasses, it will be found that the outlines of the two holes will generally 

 be seen with unequal distinctness. To most eyes the red outline will appear 

 quite distinct, while the blue figure will seem much blurred. To a few indi- 

 viduals the blue figure appears the more distinct, and these will generally be 

 found to be hypermetropic. 



Fig. 138.— Model to illustrate astigmatism. 



Astigmatism. — The defect known as astigmatism is due to irregularities 

 of curvature of the refracting surfaces, in consequence of which all the rays 

 proceeding from a single point cannot be brought to a single focus on the 

 retina. 



Astigmatism is said to be regular when one of the surfaces, generally the 



