1898.] Mr Wilson, On the production of a cloud, &c. 393 



Trans., 160, p. 333, 1870) and by Aitken (Edin. Trans., 39, I. 

 p. 15, 1897) by the action of light on various vapours, are due to 

 the ultra-violet rays alone ; for if a thin sheet of glass or mica 

 (substances which are opaque to these rays) be interposed, not a 

 trace of fog or rain is formed even when a high degree of super- 

 saturation is brought about by expansion. It is possible that the 

 small particles to which the blue of the sky is due are the result 

 of this action of the ultra-violet rays, of which sunlight, when it 

 first enters our atmosphere, doubtless contains a plentiful supply. 



(4) On the use of logarithmic coordinates in Physics. By 

 Mr J. H. Vincent. 



(5) On the Diffuse Reflection of Rontgen Rays. By J. J. 

 Thomson, M.A., F.R.S. 



It is well known that when Rontgen rays fall on a solid or 

 liquid surface there is a diffuse return of rays from the surface ; 

 these diffusely returned rays we shall call secondary rays. 



The consideration of the nature and manner of production of 

 these secondary rays suggests some interesting questions, as the 

 experiments of Sagnac and Langevin show that these secondary 

 rays differ in their properties from the primary rays which excite 

 them. So that it would seem that bodies exposed to Rontgen 

 rays can themselves emit rays which possess many properties in 

 common with the ordinary Rontgen ray. Thus for example 

 Rontgen has recently shown that air through which Rontgen rays 

 are travelling sends out rays into regions carefully screened from 

 the direct action of the primary rays. 



Sagnac, who has recently investigated the properties of the 

 secondary rays diffusely reflected from metals, finds that though 

 they resemble the primary rays in not being refracted, they are 

 absorbed with very much greater rapidity, the secondary rays 

 being practically extinguished after passing through a centimetre 

 or so of air at atmospheric pressure. Sagnac found too that the 

 penetrating power of the secondary rays depended on the nature 

 of the metal from which they emanated; thus close to the surface 

 of the metal the secondary rays were more abundant with zinc 

 than with copper, at a short distance from the surface of the metal 

 the proportions are reversed, showing that the rays from copper 

 have more penetrating power than those from zinc. Sagnac 

 arrived at his results by using the photographic method. Langevin 



