52 J^. W. Very — Nebulosity around Nova Persei. 



of its forms, whether as developed by Kapteyn* or by Seeligerf 

 or by Hinks, presupposes an albedo which is quite impossible 

 in nebulous material as it actually exists in extreme rarefac- 

 f action. Professor H. H. Turner;}: has given the following 

 rough but simple calculation in regard to Kapteyn's hypoth- 

 esis : "Light takes 8 min. to reach the moon from the sun, 8 

 months to reach the nebula from Nova Persei. Taking the 

 original flare up to be 5000 times the brightness of our sun, 

 the illumination of the nebula should be to that of the moon 

 as 5000 to (30X24X60)'. The moon could be photographed 

 with Mr. Ritchey's instrument in (say) 0^'003. Hence the 

 nebula, if of the same albedo as the moon [could be photo- 

 graphed] in 0^-003 X (30X24X60)'^ 5000 = 20 min. say. The 

 actual exposures required," says Professor Turner, " thus seem 

 reasonable on Kapteyn's hypothesis." Let us see what this 

 " reasonable " conclusion implies. It will be noted that in 

 forming his equation. Professor Turner has introduced the 

 proviso of equal albedo in nebula and moon. The assumption 

 remains in the final conclusion, and how far it is from the 

 truth may be seen from the following considerations : In 1895, 

 I measured the light from a small area of the moon in total 

 eclipse,§ photometrically, and found it to be four one-thou- 

 sand-millionths of the light from a corresponding fraction of 

 the full moon. Mv range of vision embraces lights which are 

 in the ratio of 1 to" 1,000,000,000,000,000. Hence I could cer- 

 tainly see a small luminous surface considerably fainter than 

 the fraction (less than 1 per cent) of the eclipsed moon, and as 

 no one has been able to see the nebula, I conclude that its 

 light must be less than (me one-thousand-millionth of that from 

 an equal area of the full moon. Professor Turner's calcula- 

 tion demands equal albedo, that is, equal intrinsic brightness 

 from the same angular area in the two cases. We are not con- 

 cerned with the albedo of the individual reflecting particles in 

 the nebula. This may be exactly the same as the moon's 

 albedo, but that is not the question. The albedo of the nebula 

 is the e£Fective albedo of a surface, apparently continuous, but 

 in reality undoubtedly composed of excessively minute parti- 

 cles separated by large vacant spaces. AVith the actual effec- 

 tive nebulous albedo of 10"^ (moon -■ unity), we see from 

 Professor Turner's own figures that the conclusion which he 

 draws is not warranted, and that no reflection from nebulous 

 material at the vast distance of these nebulous forms from the 

 central illuminating body can affect the photographic plate, 



* Astronomisclie Nachrichten, No. 3756, clvii, 201, Dec, 1901. 



f Astrophysical Journal, xvi, 187, 1902. 



ixhe Observatory, Feb., 1902. 



§ ** Photometry of a Lunar Eclipse," Astrophysical Journal, ii, 293, 1895. 



