GEOPHYSICAL LABORATORY. 171 



In the quantitative classification the rock falls in the as yet unrepresented 

 position 1.9.1.1, and the name italite is given it in the current classification, the 

 name leucitite being already in use. 



Chemical analysis (made on 0.0639 gram) of the garnet showed that it is a 

 titanium-rich andradite, Hke those of other Italian lavas. The very high re- 

 fractive index (1.94) is analogous to those of other titanium-rich garnets, 

 whose refractive indexes were also detennined by Dr. Merwin, of this Labo- 

 ratory. 



(17) Suir italite: un nuovo tipo di roccia leucitica. Henrj' S. Washington. Atti Accad. 



Lincei, 29, 424-435 (1920). 



An Itahan translation of "Italite: a new leucite rock" (Amer. J. Sci., 50, 

 33-47, 1920). Reviewed under No. 16 above. 



(18) Dispersion in optical glasses: I. Fred. E. Wright. J. Opt. Soc. Amer., 4, 148-159 



(1920). (Papers on Optical Glass, No. 27.) 



A convenient graphical method for illustrating the relations between differ- 

 ent types of optical glasses is to plot for each glass its refractive index, Ud, 



. , ,, ,. rio-n.^' 



against the ratio . 



nG' — np 



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, chat in a series of optical glasses the partial dispersions are related 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 wdthin 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 automatically. 

 Thus, a series of standard dispersion-curves can be set up independent of the 

 absolute refractive index. This means that if for any substance two refrac- 

 tive 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 compu- 

 tation purposes. 



(19) Dispersion in optical glasses: II. Fred. E. Wright. J. Opt. Soc. Amer., 4, 195-204 



(1920). (Papers on Optical Glass, No. 29.) 



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

 \'isible spectrum, the substitution in a dispersion formula of the reciprocal of 

 the refractive index, or of the excess refracti\'ity, 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, 

 n = A-{-B-\~^ or n—l=A'-\-B-\~-, the reciprocal of the refractive index 

 or of the excess refractivity be written: n~^ = C'-\-D-\~^ or (n— 1)"^ = 

 C-t-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 



