112 PEACTICAL PHYSIOLOGY 



CHAPTER XXVIII. 



THE OPTICAL DEFECTS OF THE EYE. 



1. Spherical Aberration. This is probably of little consequence in 

 the eye, as the action of the iris eliminates it largely. 



2. Chromatic Aberration. Eays of coloured light are refracted 

 differently according to their position in the spectrum. Those of 

 shorter wave length, as the violet and blue, come to a shorter focus 

 than do those of longer wave length, as the red. * 



EXPERIMENT I. Look through the upper part of a window towards 

 the sky. Pass a card before the eye with the edge parallel to the 

 upper side of the window frame. If the card be passed from below 

 upwards, when it has covered about half the pupil the frame will 

 be seen to have a border of blue. If the card be passed from above 

 downwards, when it covers half the pupil the edge of the frame will be 

 seen to have a reddish-yellow fringe. In the first case the less refracted 

 red constituents of the margin of the white light are cut off by the card, 

 in the second case the more refracted blue rays. 



EXPERIMENT II. Look at the incandescent filament of an electric 

 lamp. Pass a card across the pupil with the edge parallel to the 

 filament. When the edge of the card is almost covering the filament, 

 the filament is seen to have a red fringe on the side nearer the card, 

 and a blue fringe on that more remote. 



EXPERIMENT III. Von Bezold's experiment. Draw a series of 

 vertical parallel bars one millimetre wide, separated from one another 

 by a width of one millimetre. Look at these lines with imperfect 

 accommodation. If the figure be placed about 60 cms. away, and 

 the eye accommodate for an object (e.g. a pin) about 20 cms. away 

 from the eye, the white intervals may become a rather dark red, 

 and the black bars appear a bright greenish yellow. 



3. Astigmatism. It is frequently the case that the curvature of the 

 cornea, or lens, in the vertical meridian is greater than that in the 

 horizontal meridian. Therefore, accommodation for a horizontal bar 

 at a certain distance means under-accommodation for a vertical bar 

 at the same distance. Persons who have such a spoon-shaped cornea 

 are said to suffer from regular astigmatism. 



The cornea, or lens, may have irregular curvatures in various 

 meridia, resulting in irregular astigmatism. 



EXPERIMENT I. Adopting the method of ascertaining the near 

 point of accommodation in Schemer's experiment (Experiment IV., 



