Chemistry and Physics. 471 



base in Planck's formula for the intensity of radiation from an 

 ideal black body, and o- is the multiplier of the fourth power of 

 the absolute temperature as it occurs in the Stefan-Boltzmann 

 law. The numerical values employed in Henning's compu- 

 tations were: B ± = 109677-69 ± 0-06, B 2 = 10972214 ± 004, 

 .4.! = 1-008 (the atomic weight of hydrogen), A 2 = 4-002 dz 

 0-002 (at. wt. of helium), c = 2-9989 X 10 10 (velocity of light), 

 R = 8-315 X 10 7 (molecular gas-constant), and e = 4-774 X 10 10 

 (electronic charge; Millikan). The final results were c = 

 1-432 ±0 005 cm. degree, and o- =(5-717 db 0052) X 10 5 erg. 

 cm/ 2 sec/ 1 degree" 4 . For sake of comparison, the weighted mean 

 values of the best direct determinations are given as c 2 = 1-43 ± 

 0-01 and o- =(5-67 ± 0-18) X 10' 5 . The author concludes with 

 the remark that: "The splendid agreement between calculation 

 and observation lends additional support to the hypotheses of 

 the quantum theory." — Verh. d. Deutsch. Phys. Gesell., 9/12, 

 81, 1918. h. s. u. 



7. Indices of Refraction for X-Rays. — Up to the present 

 time all experimental attempts to detect the refraction of vari- 

 ous bodies for X-rays have failed signally, doubtless because the 

 indices of refraction differ from unity by extremely small 

 amounts. The question as to whether it is possible to determine 

 these indices experimentally is discussed briefly in a paper by 

 A. Einstein. 



The author's attention was drawn to this problem by some 

 photographs that were sent to him by A. Kohler and that exhib- 

 ited a peculiarity which is said to have baffled explanation. 

 "The positives, — chiefly representing human limbs, — showed at 

 the contours bright borders about 1 mm. wide in which the 

 plates appeared to have been illuminated more strongly than 

 in the surrounding regions outside of the geometrical shadows." 

 Einstein expresses the opinion that the phenomenon in question 

 is due to total reflection. Incidentally, it may be remarked that 

 he does not mention the possibility of secondary radiation. In 

 any event, the following considerations of Einstein merit the 

 attention of experimental physicists. 



The difficulty in determining n depends upon the fact that, 

 according to the classical theory of dispersion, n—1 should be 

 of the order of 10 6 . It is however easy to see that at almost 

 grazing incidence total reflection of a detectable amount must 

 obtain. Suppose n<l, say n = 1-e. Using the complements 

 of the angles of incidence and reflection, Snell's law takes the 

 form cos ^ = n cos i//. Retaining only the first two terms of 

 the series expansion of the cosine, and substituting 1-e for n, 

 it follows that e = %(^ 2 -^r' 2 ). The limiting angle \p of total 

 reflection is determined by the condition if/' = 0, hence \j/ = 

 V(2e) approximately. If e is 10" 6 then i}/ will be of the order 

 of 0-0014 radian, or about 5 seconds of arc. This magnitude 



