Notices respecting New Books. 533 



and substitute for the two spheres the ellipsoid 



a=±R+r=2-277, b = r = 1-518 . 10" 8 . 



This gives for the semidiameter of the molecule NO, by 



(22 a), 



- . 3-036. 10" 8 n 1-518 



p = i°= Fe, cos #=2 T 277' 



that is, 



J<r= 1-840. 10" 8 (28} 



The free-path semidiameter of this molecule is, according to 

 Jeans (loc. cit., p. 341), 



Lo-=1-86.10- 8 (J.) 



The coincidence is almost perfect. It thus serves also as an 

 additional corroboration of the correctness of our atomic 

 refractivities of nitrogen and. oxygen and of our estimate of 

 their atomic radii. 



Substituting the above value of s into (5), the molecular 

 ref ractivity of nitric oxide can be calculated for any X, within 

 the prescribed region at least. Since, however, the only 

 observed ref ractivity is that for sodium light, the calculation 

 can be omitted for the present. 



The purpose of this Section has been chiefly to illustrate 

 the application of the general formula (5) for diatomic 

 gases. In order to be able to treat further, and more vital, 

 concrete examples we shall have first to enlarge our meagre 

 list of atomic refractivities, preferably by adding to the 

 above three (H, 0, N) that of carbon which we shall attempt 

 to derive, with such rigour as is possible, from the optical 

 properties of diamond. But before doing so it will be 

 necessary to leave the gases and to investigate the role of 

 interaction in more dense aggregates of molecules, viz. in 

 solid crystals. The treatment, to be given in the next 

 paper, of the simplest cubic and quasi-cubic arrangements 

 of atomic centres will pave the way for some concrete and 

 more complicated problems. 



London, April, 1917, 

 Research Dept., Adam Hilger, Ltd. 



LI. Notices respecting New Books. 



Science Progress. No. 43. January 1917. 

 London : John Murray. Price bs. net. 

 r PHIS number contains several articles and reviews of interest 

 -*- to the physicist. Dr. R. A. Houstoun gives a popular account 

 of his new theory of colour vision first put forward in the Pro- 

 ceedings of the Royal Society for 1916. The popular account is 

 hardly so convincing as the Royal Society paper ; but this is no 



