ON LIGHT. 387 



The most remarkable is in the case of one variety of the 

 mineral called apophyllite which (from the peculiarity in 

 question) I have proposed to call Leucocydite, in which 

 the rings are almost devoid of colour, being merely a 

 succession of dark and light circles, much more numerous 

 than the coloured ones usually seen, and the more re- 

 mote of which, from the centre, graduate into feeble 

 shades of purplish and yellowish light. The physical in- 

 terpretation of this phsenomenon is as follows. Since the 

 colours originate in the superposition of rings about a 

 common centre, differing in diameter for the several 

 coloured rays throughout the spectrum, (as already ex- 

 plained in Lecture VII.), it follows that in this case, no 

 such difference of diameter, or but a very slight one 

 exists. Now, for crystalline plates so cut, of a given 

 thickness, the apparent diameters of the rings seen are a 

 measure of the doubly refractive energy. The more in- 

 tense this energy the closer and more compact the sys- 

 tem of rings ; for this obvious reason, that the same 

 difference of phases between the ordinary and extraordi- 

 nary pencils is developed at a less angle of inclination to 

 the axis ; and the difference of phases is a direct result 

 of difference of velocities in their internal propagation ; 

 and this again, of the doubly refractive energy. Hence 

 we conclude that in the leucocy elite all the coloured rays 

 throughout the spectrum undergo equal, or very nearly 

 equal separation at a given angle of incidence, by double 

 refraction ; and that therefore in a doubly refracting prism 

 cut from this substance, the two spectra formed by a sun- 

 beam would be of precisely equal lengths, though un- 



