Chemistry and Physics. 281 



aperture in the diaphragm and of the distribution of stars in the 

 region observed. The whole apparatus forms a sort of photo- 

 graphic telescope, mounted on a simple, jointed support which is 

 furnished with levelling screws. A coude finder, attached to the 

 apparatus, makes it possible to point towards any desired region 

 of the sky and, in particular, to center the image of a selected star 

 exactly on the opening of the diaphragm. 



Two successive exposures are required to determine the intrin- 

 sic brightness of the sky. One exposure is taken when the image, 

 formed by the telescope objective, of a comparison star is focused 

 exactly on the center of the aperture in the diaphragm, the latter 

 being cut down to the same diameter as the image. The image 

 on the photographic plate is then produced solely by the light of 

 the star. The other exposure is made with a large aperture when 

 the telescope is pointed towards the region of the sky to be inves- 

 tigated. In this case, the image on the plate is due to the light of 

 all that portion of the celestial sphere which is projected through 

 the aperture. By varying the radius of the aperture it is possible, 

 after a few r trials, to obtain the same density of negative for a 

 certain projected area of the sky as for the comparison star, the 

 time of exposure being exactly the same in both cases. It is then 

 a very simple matter to calculate the luminous intensity of one 

 square degree relative to the standard star. Fabry used Polaris 

 for reference and found that, for the non-galactic sky, an aperture 

 of diameter 3 cm gave satisfactory results with 10 minute expo- 

 sures on " Sigma" Lumiere plates. There are other ways of experi- 

 menting w 7 ith this apparatus which depend upon the photographic 

 law of reciprocity; but it seems superfluous to describe them in 

 detail because of their obvious nature. 



Fabry has tested his apparatus on two regions of the sky, one 

 near the star e Ursae Minoris in galactic latitude 30°, and the 

 other between /3 and X Cygni in the Milky Way. After compar- 

 ing his results with those obtained visually by Newcomb and by 

 Burns, he concludes that the agreement is as good as could be 

 expected under the circumstances. It maybe remarked, however, 

 that there is an unexplained lack of accord between the results of 

 statistical investigations and of direct determinations of the in- 

 trinsic brightness of the sky. — Astrophys. Jour., vol. xxxi, p. 394. 



h. s. u. 



8. Magnetische Spektren der fi-Strahlen des Radiums. — It has 

 been shown by yon Baeyer, Meitner, and Hahn that the "mag- 

 netic spectra " of the /3-rays emitted by the members of the tho- 

 rium series are, in almost all cases, discontinuous. For /3-rays 

 having speeds equal to, or less than, about 10 per cent of the 

 velocity of light the spectral bands are very well defined, but for 

 the very rapidly moving /5-rays the images are comparatively 

 indefinite and " washed out." The above named investigators 

 have since turned their attention to the /3-rays projected on the 

 occasion of the disintegration of certain members of the radium 

 series and have obtained the interesting results summarized below. 



Am. Jour. Sci.— Fourth Series, Vol. XXXIII, No. 195.— March, 1912. 

 19 



