OCTOBEE 8, 1909] 



sciea'cm: 



493 



considered a diiBculty, and astronomers have 

 welcomed the suggestion that this motion and 

 that of similar material in the solar corona 

 may be due to light-pressure. The accelera- 

 tion produced by solar gravity at the earth's 

 distance being 



0.593 _ I 



982.15 ~"icr.6 



of the acceleration by terrestrial gravity at 

 the surface of a spherical earth, the light- 

 pressure from solar radiation at this dis- 

 tance on a body of small size -whose receiving 

 surface is very large compared with its volume 

 and mass, may exert an accelerative action 

 several times that due to solar gravity, the 

 differential acceleration giving an accumula- 

 ting velocity away from the sun. 



Professor Edwin B. Frost and Mr. J. A. 

 Parkhurst find from their spectroscopic ob- 

 servations of Comet Morehouse that " par- 

 ticles of the same [chemical] constitution are 

 thrown out from the head at angles at least 

 40° asunder." ' Bredichin's hypothesis in 

 explanation of cometary tails of various tjTpes 

 requires in this case that the particles forming 

 individual component streamers of the tail 

 shall be of different sizes, rather than of dif- 

 ferent sorts, since the several chemical com- 

 pounds are not distributed unequally, but are 

 everywhere indiscriminately mingled. No 

 change of composition whatever occurs 

 through a stretch of several degrees of the 

 comet's tail, so far as can be inferred from 

 the spectroscopic evidence. 



If the masses of the particles have been 

 changed, it may be asked, will there not be a 

 change in the spectrum ? Perhaps not in this 

 case. The change of mass corresponding to 

 the velocity observed in the particles of a 

 comet's tail will be very small, according to 

 the results of Kaufmann's experiment, and 

 need not necessarily produce any sensible 

 change in the spectrum ; but at high velocities 

 approaching that of light, such changes would 

 be anticipated, and we have evidence that such 

 changes do occur. It has long been known 

 that vacuum tubes filled with various gases 

 most carefully prepared for exhibition of their 



" Astrophysical Journal, 29, 63, January, 1900. 



several spectra, suffer gradual change, the 

 spectra being eventually reduced to other and 

 simpler ones until at last possibly nothing but 

 hydrogen remains. Attempts to explain this 

 behavior on the supposition of the presence of 

 impurities, or by the occlusion of gases in the 

 walls of the tube, and the gradual absorption 

 or emanation of such residual gases, have been 

 only partially successful. The true explana- 

 tion which finally emerges is that the atoms of 

 large atomic weight are eventually broken 

 down as a result perhaps of repeated collisions 

 at high speed under electric repulsion, and are 

 resolved into simpler atoms. Sir J. J. Thom- 

 son finds that the positive rays in high vacua, 

 where great velocities have been electrically 

 produced, consist of particles of not more than 

 three sorts which are probably hydrogen, the 

 alpha particle, and helium, and that this is so 

 no matter what gas is put into the tube. It is 

 notable that the luminous emanation from 

 Nova Persei appeared likewise to consist of 

 particles of three sorts, and having masses in 

 the same ratio (1:2:4) as those observed in 

 the vacuum tube. I think, therefore, there 

 can be no doubt that the nebulosity around 

 Nova Persei was an example on an enormous 

 scale of Goldstein's " canal rays." 



The recent observations of Comet More- 

 house are not clearly decisive for or against 

 the theory of light-pressure. Barnard finds 

 that the motion of luminous knots along the 

 tail is unaccelerated. The development of a 

 comet's tail takes place at the rate of some- 

 thing like 100 km. per second, and is main- 

 tained unaltered through distances comparable 

 to that separating the sun from the earth. 

 This result was obtained in a vague way many 

 years ago by Olbers and others, but from ob- 

 servations which rested on the very uncertain 

 limit of the visibility of the extremity of a 

 comet's tail. Barnard's observation is more 

 precise, and is in agreement with the hypoth- 

 esis of a force whose acceleration diminishes 

 as a result of the motion imparted by its own 

 action. 



Next in importance to the observed velocity 

 must be placed any information which can be 

 obtained as to the size of the particles. The 



