(52G Dr. J. VY. Nicholson on the 



obtaining the desirable polish for the ready observation of 

 the effects of strain, and as the wires are of circular section, 

 the whole specimen is not in focus at one time, so that in- 

 vestigations of wires microscopically will be laborious and 

 troublesome. The main object of this paper is to point out 

 that the so-called anomalies are only special cases of the loss 

 of elasticity consequent on severe overstrain, and any theory 

 which explains one type of effect will be equally applicable 

 to this one. The effect of the first load is clearly to aid in 

 the equalization of the internal stresses in this particular 

 case- 

 in conclusion, I should like to express my indebtedness to 

 Prof. Wilberforce for his interest and helpful suggestion at 

 various stages in the work. 



George Holt Physics Laboratory 

 21 July, 1909. 





LXYI. On the Size of the Tail-particles of Comets, and their 

 Scattering Effect on Sunlight. By J. W. NICHOLSON, 

 M.A., D.Sc* 



VARIOUS opinions have been held as to the probable 

 degree of aggregation of the molecules in the tails of 

 comets. That the majority of the particles are not of 

 molecular size, in spite of the probable tenuity of the gases 

 in the tail, now seems to be fairly certain. The theory of 

 radiation pressure proposed in general terms by Arrhenius f , 

 and examined later by Schwarzschild J from a strictly 

 mathematical standpoint, demands, if it is to give a descrip- 

 tion of the phenomena associated with the tails of the most 

 difficult type, the existence of particles of such a size that the 

 pressure of solar radiation on a single particle must be about 

 nineteen times as great as the solar gravitation. This value 

 of the repulsive force from the sun, which is undoubtedly 

 present, has been shown to be necessary, apart from any 

 theory as to its origin, by the exhaustive work of Bredichin, 

 Belopolsky, and others, in a classical series of papers in the 

 Annales de V Obserixitoire de Moscon § . 



The validity of a radiation pressure theory of comets' tails 

 thus depends upon its capacity to supply a force of this mag- 

 nitude on a single particle, and also upon an experimental 

 indication that particles of the size for which this force may 



* Conimunicated by the Author. 



+ P/iys. Zeit. ii. Jahr.'Het't yi-vii. 



X Sitzwiys. der Math.-Phys. Classe zu Miinchen, 1901-2, p. 293. 



§ Tide e. g. Series ii. vols. i-ii. 



