376 VISION. LECT. XX. 



a too great curvature of the transparent cornea. In this 

 case, the rays proceeding from the point where ordinary 

 vision occurs, form their focal interval in front of the re- 

 tina ; and, hence, short-sighted persons employ diverging 

 or concave eye-glasses. These lenses increase the diver- 

 gence of the rays before they enter the eye ; and, in con- 

 sequence, an object placed at the distance of normal vision, 

 is seen under the divergence which it would have for a 

 short-sighted person, if it were brought near to the eye. 

 Convergent and divergent meniscuses, or the periscopic 

 lenses of Wollaston, correct these defects more effectually 

 than ordinary lenses. The thickness of these meniscnses 

 being necessarily less than that of the eye-glasses com- 

 monly used, they absorb a smaller quantity of light, and 

 the objects preserve more distinctness. 



Achromatism of the Eye. The achromatism of the eye, 

 which is perfect for objects situated at the distance of dis- 

 tinct vision, is owing to the circumstance that the fascicu- 

 lus which meets the retina, within the focal interval, being 

 contracted around the axis, contains rays of every colour 

 in a space too narrow for the coloured bands to be distinctly 

 formed. 



We know, indeed, that if the spaces which separate 

 images of different colours or intensity upon the retina are 

 very small, these images cannot be separately perceived ; 

 the sensation which we experience being the result of the 

 simultaneous impression of neighbouring images. 



If, then, we cannot point out exactly the cause of the 

 achromatism of the eye, we cannot, on the other hand, 

 deny that there is in the structure of its lenticular appara- 

 tus, that variety of curvatures, and of refractive and dis- 

 persive power of the media, which are the general condi- 

 tions observed in the structure of achromatic optical appa- 

 ratus. 



