G28 Dr. J. W, Nicholson on the 



it, which are the most important points in the theory, there is 

 very complete agreement. This agreement from two inde- 

 pendent modes of calculation appears to place these two 

 special points on a firm foundation. The analysis in each 

 case is somewhat intricate, and independent confirmation 

 therefore very desirable. 



It thus appears that on a radiation pressure theory of the 

 tails, particles must be present whose dimensions are com- 

 parable with the wave-length of light, and these must consist 

 of aggregates of a very large number of molecules. In view 

 of the low pressure at any point of the tail, a necessity 

 appears to arise for the presence of a large number of particles 

 of much greater size, which by continuous disintegration 

 give rise to a sufficient number of the dimensions proper for 

 a maximum radiation pressure. For an explanation of 

 hydrogen tails, this maximum must be attained very nearly 

 for a large number. The question of the origin of these 

 large particles, whether expelled from the nucleus of the 

 comet or not, may be left open for the present purpose. 



Now it seems possible to obtain certain experimental 

 indications of the distribution of particles of various sizes in 

 the tails, and the object of this note is to suggest, with 

 necessary precautions, some lines along which inquiry might 

 prove fruitful. If the particles were all small in comparison 

 with the wave-length, and with properties not greatly 

 different from transparency, the light scattered in any 

 direction would be partially polarized, and completely so in a 

 plane perpendicular to the incident rays. In such a case, 

 Lord Kayleigh's investigations in connexion with the blue 

 light of the sky would be applicable. The intensity of the 

 light scattered in any direction would be, at a distance r, 

 proportional to ma 6 /r 2 \\ where a is the radius of a particle 

 and m the number of such particles. But when larger 

 particles are present, in the number required for Schwarz- 

 sehild's theory, the scattered light loses its character, and ceases 

 to be polarized, in the absence of polarization of the incident 

 light. The writer has recently worked out in detail the case 

 of incidence of a plane beam on a perfectly reflecting sphere, 

 which may be taken as an illustration of the scattering effect 

 to be expected. The results necessary for the present pur- 

 poses may be obtained at once by elementary considerations. 

 For if E be the mean energy per unit volume of the incident 

 beam, and if the particle be sufficiently large to throw a well- 

 defined shadow, the energy stopped by its presence in the 

 path of the incident beam is 7ra 2 E. Since the scattered 

 wave at a great distance must be spherical, and this amount 



