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 Lcucocydite, 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 jDh^nomenon 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 tlie 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 leucocydite all the coloured rays 

 tin-bughout the spectrum undergo equal, or very nearly 

 equal sepai'atioii 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- 



