148 HUXLEY MEMORIAL LECTURES 



combination seems, however, to be absent or 

 diminished in effectiveness; probably because of 

 the new stability instantly assumed by the ionised 

 atoms. 



One of the most interesting points about the 

 halo remains to be referred to. The halo is 

 always uniformly darkened all round its circum- 

 ference and is perfectly spherical. Sections, 

 whether taken in the plane of cleavage of the 

 mica or across it, show the same exactly circular 

 form, and the same radius. Of course, if there 

 was any appreciable increase of range along or 

 across the cleavage the form of the halo on the 

 section across the cleavage should be elliptical. 

 The fact that there is no measurable ellipticity is, 

 I think, one which would not on first consideration 

 be expected. 



For what are the conditions attending the pas- 

 sage of the ray in a medium such as mica? 

 According to crystallographic conceptions we 

 have here an orderly arrangement of molecules, 

 the units composing the crystal being alike in 

 mass, geometrically spaced, and polarised as re- 

 gards the attractions they exert one upon another. 

 Mica, more especially, has the cleavage phe- 

 nomenon developed to a degree which transcends 

 its development in any other known substance. 

 We can cleave it and again cleave it till its flakes 

 float in the air, and we may yet go on cleaving it 

 by special means till the flakes no longer reflect 

 visible light. And not less remarkable is the 



