CHARTING THE UNIVERSE 



Now "radiation pressure," being interpreted, 

 means the pressure of light. The most familiar and 

 striking evidence of the efficiency of this force the 

 existence of which has only quite recently been 

 revealed to the physicist is furnished by the tail 

 of a comet, which, according to the most recent and 

 most generally accepted explanation, consists simply 

 of fine particles of matter driven off from the body 

 of the comet, itself a more or less nebulous mass, by 

 the "radiation pressure" of sunlight. The familiar 

 observation that the tail of the comet always points 

 away from the sun gives obvious support to this 

 hypothesis. 



Professor Campbell's suggestion, then, amounts 

 to this: That a nebula consists wholly or in part of 

 finely divided particles of matter which are thrust 

 hither and yon in seeming defiance of the laws of 

 gravitation, by the light-pressure of myriads of in- 

 candescent stars. In due course, however, the fine 

 particles of matter become aggregated say through 

 collision and! thus become too large for the light- 

 waves to act on them effectively; for, be it under- 

 stood, "radiation pressure" can oppose gravitation 

 effectively only when acting on very minute particles ; 

 somewhat as a man can oppose it by hurling upward 

 pebbles but not boulders. 



So when the nebulous particles have sufficiently 

 aggregated they begin to fall together, under the 

 influence of gravitation, presently becoming so con- 

 centrated as to form the more or less solid body that 

 we call a star. Thenceforth this body, undergoing 

 a series of internal transformations which cause the 



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