150 EXPERIMENTAL PHYSIOLOGY. 



rays pass through the bottle, and look directly down on the 

 refracted rays. The boundaries of the light pencil are curved, 

 and not plane, surfaces because the outer rays, being more 

 refracted than the inner, intersect the latter. Move the lantern 

 nearer and note that the curvature of the surfaces increases. 

 The refraction in the eye is corrected for spherical aberration to 

 some extent by the difference in the refractive indices of the differ- 

 ent parts of the lens. The central rays pass through the part of 

 the lens which is the most dense and so they are refracted more, in 

 comparison with the rays through the periphery, than they would 

 be if all the layers were of the same composition. 



Chromatic Aberration. Light which arises from objects seen 

 under ordinary circumstances is made up of waves of different 

 lengths. In passing through the refracting media each wave length 

 is bent to a slightly different extent and this causes another error in 

 the refraction of the eye. The shorter waves of the violet end of 

 the spectrum are brought to a focus nearest the lens, the long ones 

 of the red end are least refracted, while the foci of those of inter- 

 mediate length lie between these two extremes.* 

 Experiment 57. Chromatic Aberration in Refraction by a 

 Convex Lens. Set in front of the opening of the lantern the 

 ground glass screen and the diaphragm with a 2 mm. opening. 

 Place the block holding the convex lens about 15 cm. from the 

 opening. Using a sheet of paper as a screen move it back and 

 forth in the path of the refracting light until you find the focus. 

 Note that it is not pure white, but made up of coloured bands. 

 Cover the right half of the lens with a card. The light has 

 violet fringe. Uncover the lens and move the screen a little 

 nearer to it. The disc has a violent centre and red border. 

 Move the screen beyond the focus. The colours of the disc are 

 reversed. 



Experiment 58. Chromatic Aberration in the Eye. Cover 

 with a card the outer half of the right pupil and, closing the 

 left eye, look at an electric light filament. It appears to have 

 a red border at the right and a violet one at the left. Draw a 



*For the way in which chromatic aberration is done away with in lenses of fine 

 optical instruments by using layers of different dispersive power, the student 

 is referred to his text-books on Light. There is no such adjustment in the eyr 



