172 CARNEGIE INSTITUTION OF WASHINGTON. 



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. 



(20) Classification and nomenclature of optical glass. George W. Morey. J. Opt. Soc. 



Amer., 4, 205-212 (1920). (Papers on Optical Glass, No. 28.) 



The independent development of the optical-glass industry in different 

 countries and by different manufacturers, during the course of which each 

 manufacturer hsted new glasses as they were developed under whatever names 

 seemed appropriate, has resulted in great confusion in nomenclature — a con- 

 fusion increased by a frequent lack of consistency on the part of individual 

 manufacturers. It was the purpose of the author to simphfy the nomencla- 

 ture of optical glass by the elimination of many of the special names, retaining 

 only those which, while indicating the position of the glass in the general 

 scheme, leave the precise specijfication to be made by means of the optical 

 properties. 



The system of optical-glass nomenclature proposed is based on the crown- 

 flint series of "older" glasses, i. e., barium-free glasses. The dividing-line 

 between crowns and flints is at a v-va,\\ie of 56, the approximate lower limit 

 of the lead-glasses. The crowns are divided into fluor crowns, with I'-value 

 greater than 68; borosilicate crowns, with v between 68 and 61; and ordinary 

 crowns, with v from 61 to 56. The flints are divided into extra Hght, p from 

 56 to 50; hght flints, v from 50 to 38; medium flints, p from 38 to 34; dense 

 flints, p from 34 to 31; and extra dense flints. Glasses falhng outside the 

 crown-flint series are grouped with the barium-crowns and barium-fhnts. 

 The barium-crowns have wd higher than 1.60, the Hght barium-crowns have 

 no between 1.60 and 1.54, The barium-flints are divided into hght barium- 

 fhnts, riD less than 1.60; medium barium-flints, index between 1.60 and 1.64; 

 and dense barium-flints. 



It is emphasized that optical-glass type names are for convenience in casual 

 reference only, and do not imply a specification of optical properties. When- 

 ever reference is made to a definite glass, as opposed to the broad general type, 

 whether in the purchase of glass or in discussion of properties or uses, thp ref- 

 erence should be made exact by specifying the optical properties, usually the 

 refractive index for sodium light and the v-value. 



(21) The binary system akermanite-gehlenite. J. B. Ferguson and A. F. Buddington. 



Am. J. Sci., 50, 131-140 (1920). 



The binary system akermanite (2CaO.Mg0.2Si02)-gehlenite (2CaO.Al2 

 03.Si02) was studied by the quenching method and the solidus and liquidus 

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

 tions with a minimum melting-point about 70° below the melting-point of 

 akermanite, the component of lower melting-point, at a composition of about 

 74 per cent akermanite and 26 per cent gehlenite. 



The densities of crystals and glasses of akermanite, gehlenite, and several 

 intermediate mixtures were^ determined and found to confirm the isomorphous 

 character of the system. Akermanite was found to show the unusual feature 

 of its glass having a greater density than the corresponding crystals at 25° C. 

 This pecuhar character is checked by the indices of refraction for glass and 

 crystal respectively, the index of refraction of the glass being greater than the 

 maximum index of the crystal. 



The optical characters of the crystals are a continuous function of the 

 composition. In optical characters akermanite is positive and gehlenite is 

 negative. Crystals of certain intermediate compositions are isotropic for 



