182L] Cri/stallized Bodies on Homogeneous Light, 165 



especially in the extreme red and violet rays, both of which 

 would be copiously, and indeed almost entirely absorbed in 

 their passage through two plates of tourmaline of a yellowish- 

 green colour. Much more exact and unexceptionable measures 

 will be presently given, but these are quite sufficient to establish 

 the reality of the phenomenon described. 



V. Of a secondary Cause of the Deviation of TintSy subsisting in 

 certain Crystals, and of the anomalous Tints of tlie Apophyllite, 



To determine the dispersive power of any medium, and obtain 

 some rough knowledge of its law, we make a prism of it act in 

 opposition to one of a standard substance. To ascertain the 

 total dispersion of the axes of a crystal, or the angle by which 

 the extreme red and violet axes differ, we may make it act 

 against itself. Since the violet rings are more elevated by 

 refraction than the red, from the situation in which they would 

 appear to an eye immersed in the medium, a plate may be con- 

 ceived cut in such a direction as to make their apparent centres 

 coincide, in which case the tints immediately about the poles 

 will coincide with Newton's scale, and the extraordinary image 

 will totally disappear in the pole at an azimuth 45°. This condi- 

 tion gives 9 = 0, fi ~ 8 fl + § <p = 0, whence (supposing R, R' 

 the indices of refraction for extreme red and violet rays, and 5 R 

 = R' - R) we find 



Ja = S<p= — . tan. (p 



The angle tp however becomes imaginary, and this method, in 

 consequence, inapplicable when the separation of the extreme 

 axes il a) is greater than the maximum dispersion of the colours 

 of an intromitted white ray, that is, when 



SR 



I a > ; r 



R . 'Z R« _ 1 



Let us resume our equation (6), and supposing the form of 

 the function 4^, and the constants «, /r, A:', R and I R ascertained, 

 let the angle fi, at which the coincidence takes place be observed, 

 and the value of 8 a will then become known. If we suppose it 

 small, which it is in the generality of crystals, we get 



5 a = ^^— ^ ; (r) 



(4/ being put for -^ (5, d^ for the sake of brevity). At incidences 

 nearly perpendicular, 8 <^ may be neglected, and the expression 

 reduces itself to 



d^' d 

 , The comparison of these formulae with observation, which 



