ABSTRACTS 



Authors of scientific papers are requested to see that abstracts, preferably prepared 

 and signed by themselves, are forwarded promptly to the editors. The abstracts should 

 conform in length and general style to those appearing in this issue. 



OPTICS. — Dispersion in optical glasses: II. Fred H. Wright. Journ. 

 Opt. Soc. Amer. 4: 195-204. 1920. (Geophysical Lab. Papers on 

 Optical Glass, No. 29.) 



In this paper proof is given that, because of the relatively short range of the 

 visible spectrum, the substitution in a dispersion formula of the reciprocal of 

 the refractive index, or of the excess refractivity, or by analogy of other func- 

 tions of the refractive index for the direct values, leads to dispersion formulas 

 which are fairly satisfactory. Thus, if in the two-constant Cauchy formula, 

 w = A + B-X"^orw— 1 =A' + B •X"^, the reciprocal of the refractive index 

 or of the excess refractivity be written: m~^ = C' + D-X~- or (w — 1)"^ = 

 C + D-X~^, the new equations represent rectangular hyperbolas in case X"^ is 

 considered to be the independent variable. The last equation was recently 

 suggested as a substitute for the Hartmann dispersion formula. A series of 

 computations demonstrates, however, that for the crown-glasses this equation 

 is less satisfactory than the Cauchy formula, while for the flint-glasses none of 

 the foregoing equations is especially good. The last equation is, moreover, 

 always less satisfactory than the Cauchy formula for computation purposes. 

 The usefulness of the last equation appears, therefore, to be limited and less 

 satisfactory for general application than the two-constant Cauchy formula. 



F. E. W. 



GEODESY. — Modern methods for measuring the intensity of gravity. Clar- 

 • Ence; H. Swick. U. S. Coast and Geodetic Survey Special Pub. 69. 

 Pp. 96, figs. 26. 1921. 



The measurement of gravity on land by the U. S. Coast and Geodetic 

 Survey is made with an accuracy of about 2 or 3 parts in a million. Such 

 accuracy with a field instrument requires more than ordinary care and skill 

 on the part of the observer. At the present time all gravity measurements 

 are made with pendulums only a quarter of a meter in virtual length, which 

 are placed in an air-tight receiver and swung under a pressure of about 1/15 

 of an atmosphere. The cost and time required for a determination are very 

 much less than they were and the results obtained are far more accurate 

 than with the old types of apparatus. 



This publication is primarily a working manual for field and office use. 

 Aside from one chapter of historical data the entire book is given over to a 

 detailed description of the instruments and to instructions for making the 

 observations and computations. All the necessary formulas and tables 

 are included. Sample records are shown and a sample computation for 

 one station is carried through all the required steps. 



INORGANIC CHEMISTRY.— r/j^ binary system dkermanite-gehlenite. 

 J. B, Ferguson and A. F. Buddington. Amer. Journ. Sci. 50: 

 131-40. 1920. 

 The binary system akermanite (2CaO.Mg0.2SiO)-gehlenite (2CaO.Al203- 

 SiOs) was studied by the quenching method and the solidus and liquidus 

 curves were determined. The system forms a complete series of solid so- 

 lutions with a minimum melting-point about 70° below the melting-point 



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