Chemistry and Physics. 95 



an elevation of 9000 meters 2/3 of the total air mass is left below. 

 Again, the relatively cool outer atmosphere of the sun may absorb 

 all radiations of wave-length shorter than about 289^. The most 

 plausible explanation of the existence of an approximately abrupt 

 ultra-violet limit to the solar spectrum seems to be given by 

 Rayleigh's theory of the scattering of short waves by molecules 

 and small suspended particles, which accounts for the blue color 

 of the sky, etc. To test this point, Wigand has done some pre- 

 liminary work by focusing the bluest part of the skylight on the 

 slit of the spectrograph. Four different negatives gave the fol- 

 lowing limits: 289*87, 289*80, 289*60 and 289*39 W. It is thus 

 evident that the limit is at least as low for blue skylight as for 

 direct sunlight, as would be expected if the true explanation is to 

 be found in the scattering by small particles. — Phys. Zeitschr., 

 Nov. 15, 1913, p. 1144. h. s. u. 



9. Change in a Lunar Crater. — Most, if not all, of the 

 changes which have been observed in the moon and accepted as 

 thoroughly verified are of a periodic nature. A marked case of 

 a non-periodic change is recorded in a short article by William 

 H. Pickering. The author says: "It is indeed the most marked 

 non-periodic change that the writer has ever detected." The 

 crater in question is named Eimmart and is situated at the north- 

 west border of Mare Crisium, in latitude +24° and longitude 

 295°. This crater is about 40 kms in diameter. The general 

 nature of the phenomenon observed is that while formerly, at 

 each lunation, the crater apparently filled up gradually and over- 

 flowed with a white material, the source of which was a point at 

 the foot of the northern interior slope, this change no longer 

 occurs. The last "eruption" observed occurred in January, 1913. 

 Only one observation was made in February and March, but less 

 activity was then shown, while in April and May the phenomenon 

 was so reduced in intensity as to be scarcely noticeable. Inter- 

 esting drawings of the appearance of the crater during and after 

 its maximum activity are reproduced in the paper, but the author 

 does not discuss the geological or physical aspects of the results 

 obtained with the Draper refractor. — -Astron. ISTachrichten, No. 

 4*704, p. 414. h. s. u. 



10. Propagation of Light in Dispersing. Media. — According 

 to the Principle of Relativity it is impossible for a velocity to 

 exceed that of light in free space. This applies to the convec- 

 tive velocity of an electron or ordinary body as well as to the 

 velocity of propagation of a mechanical or electrodynamical sig- 

 nal. On the other hand, it has often been shown, experimentally 

 that, in the immediate vicinity of an absorption band, a region 

 exists throughout which the index of refraction is less than unity. 

 Consequently the "velocity of light" (no matter whether this 

 term is taken to mean the group velocity or the phase velocity) 

 in such a region is greater than c, the velocity of light in v actio. 

 This apparent contradiction between experiment and theory has 

 been exhaustively discussed and satisfactorily explained in two 



