376 THIRD REPORT 1833. 



subject, suggests that a compensation may take place between 

 the refractions at the cornea and the crystalline. 



Several writers observe, that the sensible achromatism is 

 difficult to understand, because the refractions are all performed 

 one way. 



With a view to the theory, it is to be observed that although 

 the refractive powers of the several media of the eye have 

 been accurately determined, and the dispersive indices of the 

 aqueous and vitreous humours, yet no observations of the se- 

 paration of the different prismatic rays in these media have 

 been given, and none for the dispersion of the crystalline lens. 



The author has accordingly tried such experiments by form- 

 ing portions of these media (from the eye of an ox) into prisms 

 between glass plates, and measuring the proportions, both of 

 the whole extent, and of the several parts, of the spectra which 

 they produced. 



In the vitreous humovu" this was easily done, as the whole 

 spectrum was very distinct, and the principal lines well defined. 

 With the crystalline it was extremely difficult to make the ob- 

 servations satisfactorily, and the results are less certain. 



The measurement was made micrometrically by the rotation 

 in its own plane of a plate-glass prism of very small angle, so 

 as to give an image of the spectrum sensibly unaltered, but 

 deviated in position, and any definite part of this image could 

 be brought by rotation into coincidence with any part of the 

 fixed image : the angular separation of the diflPerent parts or 

 lines of the spectrum could be thus readily deduced from the 

 arc of rotation observed. 



For comparison of the prismatic dispersion of the vitreous 

 and crystalline, the mean results of several sets of observations 

 were as follows: 



Angle of the prism, 75° 



As far as such determinations can be relied on, it hence ap- 

 pears : 



1st, That the dispersion of the several portions of the ci'y- 

 stalline are nearly equal. 



2ndly, That the separation of the different rays in the cry- 

 stalline is greater than in the vitreous humour ; but 



