Size of the Tail -particles of Comet*. (>27 



be obtained are present in large numbers in the tail, so that 

 the effect of radiation upon these large particles shall bo 

 predominant. 



In Schwarzschild's paper, the pressure produced by a 

 plane wave train impinging on a perfectly reflecting sphere 

 placed in its path is examined, in order to obtain an upper 

 limit to the possible pressure. For it is evident th-it in this 

 cisc the pressure must be a maximum for a given dimension 

 of obstacle. When the diameter of the sphere is not greater 

 than about a quarter of the wave-length of light, assumed to 

 be monochromatic, Schwa rzschild finds that the pressure is 

 given by the formula 



P = 2247r 5 a 2 E/3\ 4 



within an accuracy of 20 per cent., where E is the energv 

 per unit volume of the incident beam, and a and A, the radius 

 of obstacle and the wave-length respectively. Adopting tbe 

 specific weight of unity for the material, and taking the 

 energy density of the solar radiation at the surface of the sun * 

 to be 27*5 10~ 4 grm. cm. -2 , with a gravitation constant at 

 tbe sun's surface of 27'5 times that of the earth, the ratio of 

 pressure to solar gravitation becomes 3/4a, when a is in //,//,, 

 and equal to \/8. The two forces are thus of the same order 

 of magnitude. Their comparison at the surface of the sun 

 instead of in the comet, is legitimate, since both obey the 

 inverse square law. 



But when the diameter of the obstacle increases, Schwarz- 

 schild finds that the pressure rises rapidly to a maximum, 

 and becomes nearly twenty times the weight for a particle 

 whose diameter is about one-third of the mean wave-length 

 of visible light. 



Account has now been taken of the distribution of energy 

 among the different wave-lengths in the spectrum, by the use 

 of Wien's law. When the diameter increases still further, 

 the pressure sinks to the value ira-E valid for very large 

 obstacle-. 



The writer has recently repeated these calculations of 

 radiation pressure by a shorter analysis, which may be readily 

 adapted for rapid numerical calculation. The arithmetical 

 results t are not in complete accordance with those of 

 Schwarzschild, but as regards the maximum value of the 

 radiation pressure, and the size of particle corresponding to 



* Arrhenius, he. cif. 



~ These results will shortly be presented to the Royal Astronomical 

 Societv. 



2 S 2 



