530 merwin: measueement of refractive index 



and for Avogadro's constant or the number of molecules per 

 gram molecule 



N = (6.05 ± 0.04) X 1023. 



The agreement of the above value for no with what is perhaps 

 the best value from other methods, (2.705 ± 0.005) X 10^^ 

 (Millikan), must give weight to the accuracy of the estimation 

 of the atmospheric losses in the determinations of the solar 

 radiation by the Smithsonian Observatory. Some criticism is 

 made of the procedure of L. V. King who uses our observed 

 moist-air transmission coefficients for a similar purpose, mainly 

 in that he uses coefficients for wave-lengths where selective 

 absorption is present and for such losses Rayleigh's formula 

 does not hold. 



A remnant of the volcanic dust from the eruption of Mount 

 Katmai, Alaska, in 1912, scattering somewhat less than 3 per 

 cent of the incident radiation, is indicated by the 1913 trans- 

 mission coefficients. It is perhaps worth noting that, fine as 

 this dust must be to have remained suspended in the upper air 

 over a year, its scattering of radiation scarcely varies with the 

 wave-length, at least between the limits 0.38/x and 0.81/x. 



PHYSICS. — Measurement of the extraordinary refractive index of 

 a uniaxial crystal by observations in convergent light on a 

 plate normal to the optic axis. H. E. Merwin, Geophysical 

 Laboratory. 



During an investigation of the optical properties of the tri- 

 oxides of arsenic and antimony, it was found that attempts to 

 crush the tabular, hexagonal crystals into grains which could 

 be suitably oriented for the determination of the extraordinary 

 refractive index, resulted in producing aggregates, probably due 

 to gliding. The possibility of measuring e by a study of the in- 

 terference rings on basal plates in convergent polarized light 

 was then considered. 



The figure represents a vertical section through a basal plate 

 of an optically negative uniaxial crystal. The wave-normals 

 for the ordinary and the extraordinary wave originating from the 

 same incident wave are shown. They form with the optic axis 



