388 ON LIGHT. 



equally refracted, or that the highest index of refraction 

 would 'be accompanied with the least dispersive power. I 

 have not made the experiment, but that such would be 

 the case there can be no doubt. In the spectra formed 

 by an Iceland spar prism, the reverse is the case the 

 higher refractive index corresponding to a much higher 

 dispersive power, and the most refracted spectrum being 

 much longer and much more brilliantly coloured than 

 the least. 



(164.) Another highly remarkable example of this kind 

 is found in the mineral called Vesuvian, a uniaxal crystal 

 of a greenish hue, which to a certain degree interferes 

 with the vivid development of its coloured rings. It does 

 not, however, prevent their being well observed and 

 they present this very singular anomaly, viz., that the 

 system of rings formed by the red rays is considerably 

 smaller than those formed by the violet, and in conse- 

 quence that the order of tints in the rings formed in 

 white light is inverted, so that, of the spectra formed by 

 a prism of this substance, the more refracted ought to be 

 the shorter, and the least coloured. This kind of ano- 

 malous action is, however, carried still further in another 

 variety of uniaxal apophylite, in a plate of which perpen- 

 dicular to the axis, rays of a medium refrangibility/^rw 

 no rings at #//, so that for such rays the substance is singly 

 refractive. Proceeding from this medium refrangibility 

 towards either end of the spectrum, rings are formed, 

 contracting in diameter, as the red or violet end is ap- 

 proached, but most rapidly towards the red. It would 

 not be too much to expect that if a prism could be 



