Size of the Tail-particles of Comets. 629 



of energy is radiated off, the amount crossing unit area at 

 distance r must be 7ra 2 E/47n i2 or Ea 2 /4r 2 , in a first order 

 approximation. The corresponding result for a number of 

 such particles follows at once if, as seems certain, their 

 distances apart are large multiples of the mean diameter. In 

 other words, the shadow cast by one particle is supposed not 

 to fall on the others. This assumption is borne out by such 

 observations as have hitherto been made. For example, 

 Rosenberg *, from a series of observations of the surface 

 brightness of Comet Morehouse (1908 C) , has concluded that 

 the material is in so extreme a state of tenuity that there can 

 be no question of the casting of shadows. The Lommel- 

 k^eeliger law of diffuse reflexion was used in the deter- 

 mination. Moreover, it has long been a matter of visual 

 experience that comets do not greatly darken a bright star 

 when passing over it. In addition to its indication of an 

 absence of occupation of one particle by another, this fact 

 affords valuable evidence of the smallness of the absorption. 

 But in the presence of sodium vapour, or of other vapours 

 which may possess similar properties, great care must be 

 taken in the interpretation of results of experiments on 

 brightness, after the remarkable researches of Wood f on 

 the extent to which absorption is affected by fluorescence. 



The need for a determination of the polarization of the 

 scattered sunlight is therefore evident. In the case of Comet 

 Morehouse, Rosenberg has recorded that no visual effect of 

 polarization could be detected, but, as he remarks, the reflected 

 light was in this case only a very small fraction of the total 

 light, at any rate within the limits of the visible spectrum. 

 As exact a determination as possible of the proportion of 

 scattered light which is polarized would go far towards a 

 determination of the average size of the particles, and 

 experiments on these lines do not appear to have been per- 

 formed as yet J. 



The absence of any great amount of polarization in the 

 total light affords no strong indication that the proportion of 

 reflected light is small. Rosenberg § appears at one point to 

 regard this as a possible corollary, but, as we have seen, this 

 can only be the case in the absence of a large number of 

 reflecting particles too great in magnitude to polarize the 

 scattered light. If, when the intrinsic light of the comet and 



* Astro physical Journal, Nov. 1909. 

 t Phil. Mag. Dec. 1908, Oct. 1909, and other Papers. 

 X I find that Prof. H F. Newall has designed a series of experiments 

 on these lines, in view of the approach of H alley's Comet. 

 § Loc. cit. p. 268. 



