1906.] on Eclipse Problems and Observations. 285 



Lengths measured along the horizontal axis indicate the sizes of 

 the particles. If the particle has a diameter equal to one-tenth of 

 the wave-length of light, then it is attracted by the mass of the sun 

 with a force greater than that exerted by the light of the sun in re- 

 pelling it. If the particles have diameters about one-third of the 

 length of the wave of light, then those particles are driven away. 

 The particles of diameters equal to one-third of the wave-length will 

 be repelled with a force which is twenty times as great as the force 

 of the sun's attraction. When we get a particle of the size of the 

 wave-length, it is repelled with a force about four times that of the 

 attraction produced by gravity. When we get a particle of diameter 

 equal to about two-and-a-half wave-lengths of light, then it is not 

 more repelled than it is attracted. Particles smaller than that dot on 

 the scale of my diagram are necessarily drawn into the sun ; particles 

 larger than the largest circle on my diagram are also necessarily drawn 

 into the sun. Particles of intermediate sizes are repelled. 



Here, then, we have a kind of sorting process under the influence 

 of this pressure of radiation ; but we must not be misled into the 

 idea that in the neighbourhood of the sun we have nothing but these 

 small particles or these big ones, because we must remember that 

 there is a constant supply of meteors, stones, comets, and so on, 

 always attracted by the sun, and always heated up and pulverised 

 under the influence of its temperature. All that we can say is, that 

 the surroundings of the sun are factories of dust and splinters, and 

 that the sweeping process due to the pressure of light is always going 

 on, possibly more energetically from some parts of the sun's surface 

 than from others. 



But, even if there were no dust or splinters, and no supply of 

 materials for these splinters, yet still we should have emanations 

 from the sun of a kind which it has only been possible to examine in 

 quite recent years. The work of Elster and Geitel has shown that 

 in the neighbourhood of all incandescent metals there are curious 

 phenomena which, as we now know mainly by the work of that ever 

 active band of workers at the Cavendish Laboratory under the 

 inspiriting leadership of Professor J. J. Thomson, are due to the 

 emission of positive and negative ions from these bodies. 



I wish that 1 could dwell in detail on some of this work, but time 

 will not allow me to do more than say that the evidence conclusively 

 shows that these small corpuscles of Professor Thomson are emitted 

 in large numbers from incandescent solids, especially from glowing 

 carbon and glowing hme, so that Professor Thomson, in one of his 

 most recent books, says that "the fact may have an important 

 appUcation to some cosmical phenomena, since, according to the 

 generally received opinion, the photosphere of the sun contains 

 quantities of glowing carbon. This carbon will emit corpuscles 

 unless the sun by the loss of corpuscles at an earlier stage has 

 acquired such a large charge of positive electricity, that the attraction 



