CHROMATIC ABERRATION 673 



optical axis and in the periphery. Unequal refraction of these two regions 

 is called spherical aberration. It is corrected in optics by diaphragms which 

 shut out the light, either from the borders of the lens or from its center. The 

 former method is used in the eye. To demonstrate Spherical Aberration, look 

 at an object two meters from the eye, such as part of the window. Pass a 

 card across the eye until the light enters only at the margin of the pupil, 

 i.e., the borders of the lens. It will be found that the object is no longer in 

 focus and the outlines are dim and diffused. Normal eyes are near-sighted 

 for the rays that are refracted by the borders of the lens. 



13. Chromatic Aberration. Look toward the borders between the 

 sash and the bright light of an open window, at a distance of twenty feet or 

 more. Use the right eye only. Bring a card across the pupil approaching 

 from the side of the light until the eye is almost covered with the card. 

 The window sash will seem to have a blue-violet fringe. If the card is 

 brought across from the opposite side, the sash will have a reddish-yellow 

 fringe. 



Make a cross of two strips of Bradley's pure color paper, one red and the 

 other blue, on a black surface. When held at the proper distance the red 

 appears nearer than the blue. This phenomenon is brought out more strongly 

 by covering the colored papers with very thin white tissue paper. The 

 judgment of distance is based on the effort of accommodation which is 

 greater for the red than for the blue and violet rays. 



14. Schemer's Experiment. Use two needles on corks, the method 

 described in experiment i, placing one at a distance of 20 cm., and the other 

 about 60 cm. from the eye. Use only the right eye, look through two pin- 

 holes in a card at the far needle. The near needle will appear double, but 

 the images will be somewhat blurred. While looking at the far needle, bring 

 a cardboard across the right hole, note that the left image of the near needle 

 disappears, and vice versa. If one accommodates for the near needle, the far 

 needle appears double, and upon covering the right hole with the card the 

 right image of the far needle disappears. This is known as Scheiner's Ex- 

 periment. Construct a diagram to explain these phenomena. 



15. Purkinje-Sanson's Images. Examine the eye of another person in 

 a dark room as follows : With the observing eye focus for a far object, let the 

 observer hold a candle slightly to one side of the axis of vision and about 

 one foot from the eye. If the observer looks into the other eye from the side 

 opposite the candle, he will be able to see three reflected images, figures 457 

 and 458. One, from the anterior surface of the cornea, is bright and dis- 

 tinct, and of medium size and erect. In the middle of the pupil there will be 

 a second image, larger and quite dim. This is a reflection from the front 

 of the lens. The third image, reflected from the posterior surface of the lens, 

 will seem to be farther back in the eye, quite small and inverted. These 

 images can all three be seen at once with careful adjustment of the relative 



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