AUGUST 19, 1921 abstracts: optics 345 



than Cambrian, and with which the dike may possibly be correlated as 

 a late differentiate. The youngest known igneous rocks in the dis- 

 trict are dikes of Triassic diabase to which the dike cutting the pyrite 

 body bears no resemblance as it is the first of its kind to be described 

 from the Virginia Piedmont province. It cannot be older than Cam- 

 brian and may be as late as Triassic. The rock is more closely allied 

 in mineral composition and structure with the Triassic dikes of basic 

 igneous rocks than with the igneous bodies of earlier age occurring 

 in the area. For these reasons the writer regards the dike as being 

 probably not older than Triassic. 



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: I. Frkd. E. Wright. Journ. 

 Opt. Soc. Amer. 4: 148-159. 1920. (Geophysical Lab. Papers on Optical 

 Glass, No. 27.) 



If partial dispersions alone are considered and plotted one against the 

 other, the result in each case for a series of optical glasses is a straight line. 

 This fact, that in a series of optical glasses the partial dispersions are re- 

 lated by linear functions and that these functions are the same for all glasses, 

 proves that, if a single partial dispersion is given, the entire dispersion-curve 

 is fixed, irrespective of the type of glass. This means that within the limits 

 to which this statement holds, namely, about one unit in the fourth decimal 

 place, if any partial dispersion is given, all other dispersions follow auto- 

 matically. Thus, a series of standard dispersion-curves can be set up in- 

 dependent of the absolute refractive index. This means that if for any 

 substance two refractive indices be given, the dispersion-curve can be written 

 down directly; that in case two substances of very different refractive 

 indices are found to have the same actual dispersion for one part of the 

 spectrum their dispersion-curves are identical to one or two units in the 

 fourth decimal place throughout the visible spectrum. From these relations 

 it is possible to build up dispersion formulas containing two, three, or more 

 constants which represent the data in the visible spectrum with a high degree 

 of exactness. Certain of these formulas are of such form that they are 

 valid far into the infra-red and ultra-violet, but break down of necessity 

 as an absorption band is approached. Certain of the dispersion formulas 

 thus obtained are well adapted for computation purposes. F. E. W. 



GEODESY. — Elements of map projection with applications to map and chart 



construction. Charles H. Deetz and Oscar S. Adams. U. S. Coast 



and Geodetic Survey Spec. Publ. 68 (Serial 146). Pp. 163, figs. 74, 



pis. 8. 1921. 



The aim of the authors is to present in simple form some of the ideas that 



lie at the foundation of the subject of map projections. Many people have 



rather hazy notions of what is meant by a map projection, to say nothing 



of the knowledge of the practical construction of such a projection. 



