COLOURS IN METAL GLASSES AND IN METALLIC FILMS. 391 



In this case, namely, when the incident light is polarised perpendicular to the plane 

 of incidence, it further appears that if an analysing nicol be introduced so as to 

 polarise the emergent light in the plane of incidence, then the analysing nicol removes 

 the Yj component of Ej and the vanishing of Z, also, for (f> = causes a dark band 

 to cross the field over the diffraction disc if there be only one particle sending light 

 up the tube, the dark band lying along the axis of 6 in fig. 8, i.e., in the plane of 

 incidence, and this also was observed by SIEDENTOPF and ZSIGMONDY for the particles 

 in gold glass (loc. cit., p. 12). 



The discussion of the cases of figs. 4 and 5 presents no difficulty. The phenomena, 

 including the correct position of the black spot, are again explained, by means of the 

 hypothesis that the small particles are spheres. 



Thus all the phenomena, observed in the second focal plane of the microscope, due 

 to particles smaller than 0'1/x, are exactly those which would be produced by spheres 

 of metal of radius small compared with the length of a wave of light in the glass. 



If now the particles were small spheroids, or crystalline in structure, then the 

 position of the black spots, if indeed any existed, and the positions of the plane of 

 polarisation of the light emitted from the particles, would depend on the orientation 

 of the particles. Unless, therefore, the orientation of all the particles were the same, 

 we should, if many particles were sending light up the tube, get no black spot in the 

 focal plane, because the black spot, supposing there to be one, due to one particle, 

 would not coincide with that due to another. And further, even if the orientation 

 of all the particles were the same, and if every particle alone did send off no light in 

 some particular direction, so that there were a black spot in the second focal plane, 

 then, unless the common orientation were such that, for every plane of polarisation 

 of the incident light, the black spot were in the same plane as if the particles were 

 spheres, which is an impossibility, spheroidal or crystalline particles could not account 

 for the effect observed. 



These considerations show, therefore, that the small particles in gold ruby glass are 

 really spheres of gold, so long as their dimensions are considerably smaller than 0'1/x 

 (10~ 5 centim.). 



This result is of considerable interest in connection with the formation of crystals. 

 When a metal crystallises out of a vitreous solution, it appears that until the 

 dimensions have increased beyond a certain limit, the forces of surface tension 

 overcome the crystallic forces, and the particles of metal are spherical and not 

 crystalline.* 



Mr. G. T. BEILBY has arrived at the same conclusion from microscopic examination 



* [Note added Uth May, 1904. The presence of crystals, whether of silicates or of reduced metal, in 

 many pottery glazes suggests that minute spheres of the same material as the crystals were present before 

 the formation of these crystals, and that some may co-exist with the crystals. The colours of the glazes 

 may therefore be wholly or in part due to the presence of these minute spheres, in the same manner as a 

 gold ruby glass depends for its colour on the presence of minute spheres of gold.] 



