150 HUXLEY MEMORIAL LECTURES 



under the conditions of thickness obtaining in 

 rock sections. It is well known that there is in 

 biotite a maximum absorption of a plane polarised 

 light ray when the plane of vibration coincides 

 with the plane of cleavage. A section across the 

 cleavage then shows a maximum amount of 

 absorption. A halo seen on this section simply 

 produces this effect in a more intense degree. 

 This is well shown in Fig. 3 (ante) on a portion 

 of the halo-sphere. The descriptive name 

 " Pleochroic Halo " has originated from this 

 fact. We must conclude that the effect of the 

 ionisation due to the alpha ray has not been to 

 alter fundamentally the conditions which give 

 rise to the optical properties of the medium. 

 The increased absorption is probably associated 

 with some change in the chemical state of the iron 

 present. Haloes are, I believe, not found in 

 minerals from which this element is absent. One 

 thing is quite certain. The colouration is not due 

 to an accumulation of helium atoms, i.e., of spent 

 alpha rays. The evidence for this is conclusive. 

 If helium was responsible we should have haloes 

 produced in all sorts of colourless minerals. 

 Now we sometimes see zircons in felspars and in 

 quartz, etc., but in no such case is a halo pro- 

 duced. And halo-spheres formed within and 

 sufficiently close to the edge of a crystal of mica 

 are abruptly truncated by neighbouring areas of 

 felspar or quartz, although we know that the rays 

 must pass freely across the boundary. Again it 



