F. W. Very — Nebulosity around Nova Persei. 53 



even though the exposure were to be prolonged many thou- 

 sand fold. 



(3) The supposition that the observed motions are actual! j 

 those of some form of matter, either itself luminous, or pro- 

 ducing luminosity in widely distributed material with which 

 the moving substances react, demands so large an expenditure 

 of force in sustaining the prolonged movement to enormous 

 distances, that some hesitation in adopting it is pardonable. 

 Yet, considering the stupendous scale of operations in the 

 nova, the objections on this score do not seem insuperable. 

 Comet's tails have been seen to develop through millions of 

 miles in a few hours,* perhaps by electric repulsion, perhaps 

 by the pressure of light on small masses, but at any rate with- 

 out demanding greater force than the sun is competent to pro- 

 duce. High velocities have been measured in vacuum tubes 

 for the cathode rays, and yet higher ones for the luminous 

 column from the positive pole, even approaching, if not equal- 

 ing that of light. These facts at once suggest a movement of 

 Thomsonian corpuscles, or negative ions, under a magneto- 

 electric impulse, as a solution of the problem. 



Professor C. D. Perrine, however, has urged what appear to 

 him to be further objections to the hypothesis of a real trans- 

 lation of matter. He says :f " The motions observed are not 

 radial. Nearly all of them have large tangential components. 

 It is difficult to account for these tangential components. A 

 consideration of the conditions probably existing in the nebula, 

 upon the assumption of an actual translation of matter, would 

 lead us to expect a very rapid loss of light. The inner ring 

 has decreased in brightness, and some of its features have 

 become too faint to record themselves on the photographs. 

 Several masses, all in the outer ring, have been recorded only 

 on the later photographs, and have grown both in brightness 

 and size, a condition difficult to explain on the above hypoth- 

 esis. It is perhaps not inconceivable that the two rings repre- 

 sent different phenomena." If these difficulties urged by 

 Professor Perrine can be removed, nay more, if the facts con- 



* Miss Agnes M. Gierke, in her " History of Astronomy during the Nine- 

 teenth. Century" (4th ed,, p. 100), says of the comet of 1811, that Olbers 

 * ' calculated that the particles expelled from the head traveled to the remote 

 extremity of the tail in eleven minutes, indicating by this enormous rapidity 

 of movement (comparable to that of the transmission of light) the action of 

 a force much more powerful than the opposing one of gravity." By refer- 

 ence to Olbers' original communication in Zach's Monatliche Correspondenz 

 (xxv, pp. 3-22, 1812) it will be seen that the extreme length of the tail of this 

 comet, on October 13, 1811, was taken at 0'6391 astronomical units, or about 

 96,000,000 kilometers ; and the time consumed by the vapors in attaining 

 this distance, computed according to Newton's method, was stated to be 

 eleven days, instead of eleven minutes. 



f Astrophysical Journal, xvi, 260, 1902. 



