SPHERICAL ABERRATION 761 



of the candle flame as formed on the retina of the eye in the tube. In a 

 favorable experiment, a clear inverted image of the candle can be seen on 

 the retina through the semi-transparent membranes of the eye. The same 

 experiment can be demonstrated with the camera, or with a small lens, using 

 a ground-glass plate to make the image more apparent. 



12. Spherical Aberration. In physical optics it is found that it is 

 difficult to grind lenses so that they will refract equally in the center or 

 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 aberra- 

 tion, look at an object two meters from the eye, such as a part of the window 

 moulding. Pass a card close in front of 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. Step nearer and nearer the window, when quite close the 

 outlines of the moulding become clear again. 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 accommoda- 

 tion which is greater for the red than for the blue and violet rays. 



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

 scribed in Experiment 10, 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 Experiment. It depends on the diffuse stimulation of points 

 on the retina outside of the principal axis. The apparent images are 

 referred out in space along the corresponding secondary axes. Construct 

 a diagram to explain these phenomena. 



