DEFECTS IN THE OPTICAL APPARATUS 649 



very transparent; and if the surface 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 and would fall upon other parts of 

 the same membrane, producing indistinctness of the images. 



Chromatic Aberration. In the passage of light through the periphery of 

 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 undergo. This 

 is termed chromatic aberration. It is corrected by the use of lenses constructed 

 of alternate layers of glass of different refractive indices so ground that they 

 produce chromatic dispersion in opposite directions and thus mutually correct 

 any chromatic aberration which may have resulted. The human eye has 

 considerable chromatic aberration, as may readily be demonstrated, experi- 

 ment 13, page 673. 



An ordinary ray of white light in passing through a prism has its con- 

 stituent rays refracted in unequal degrees, and therefore appears as colored 

 bands fading off into each other, known as the spectrum. The colors of the 

 spectrum are arranged as follows: red, orange, yellow, green, blue, indigo, 

 violet; of these the red ray is the least, and the violet the most, refracted. 

 Hence, as Helmholtz has shown, the rays from a white point cannot be ac- 

 curately focussed on the retina, for if we focus for the red rays, the violet are 

 out of focus, and vice versa: such objects, if not exactly focussed, are often 

 seen surrounded by a pale yellowish or bluish fringe. 



For similar reasons a red surface looks nearer than a blue one at an equal 

 distance, because, the red rays being less refrangible, a stronger effort of 

 accommodation is necessary to focus them, and the eye is adjusted as if for 

 a nearer object, and therefore the red surface appears nearer, experiment 13. 



Astigmatism. This defect, which was first discovered by Airy, is due to 

 a greater curvature of the refractive surfaces of the eye in certain meridians 

 than in others. Thus vertical and horizontal lines crossing each other can- 

 not both be focussed on one plane; one set stands out clearly, and the others 

 are blurred and indistinct. This defect, which is generally present in a slight 

 degree in all eyes, is usually seated in the cornea, but occasionally in the 

 lens as well. 



The plane of greatest curvature in the cornea is usually in the vertical 

 meridian, a fact which doubtless comes from the pressure of the eyelids during 

 development. If one looks at figure 463, A or B, with one eye, the three lines 

 in the radii of the figure will be seen with unequal distinctness. Certain 

 sets will stand out sharp and black and others dim and with indistinct out- 

 lines, and if the astigmatism is great enough the three lines may not be dis- 

 tinguished. Figures C and D of this series enable one to detect minute traces 

 of astigmatism with great accuracy. 



It is somewhat difficult to picture the rays from a luminous point in their 



