Explanation of the Phenomena — Hotchkiss. 305 
AN EXPLANATION OF THE PHENOMENA SEEN IN THE 
BECKE METHOD OF DETERMINING INDEX 
OF REFRACTION.* 
By W. O. Hotchkiss, Madison, Wis. 
In the identification of minerals in rock sections the 
method of procedure is always the determination of the fixed 
physical properties. Among these constants, the ones 
which are ordinarily employed on account of their easy de- 
termination are the index of refraction, birefringence, posi- 
tive or negative character, orientation of the plane of the 
optic axes, and angle of the optic axes. Cleavage and ex- 
tinction angles are also useful when the orientation of the 
section can be ascertained with any degree of certainty. 
Among these properties the refractive index and the bire- 
fringence are probably known for the widest range of min- 
erals and are therefore most useful. The latter is readily 
determined by the method described by L. V. Pirsson and 
H. H. Robinson. f The former is measured by the method 
of Count von Chaulnes (see Rosenbusch-lddings for de- 
scription of method) and by the Becke method. The first 
method gives the numerical index but is clumsy and inaccu- 
rate ; the latter merely indicates whether the index is 
higher or lower than that of the contiguous medium, but 
it is easily applied and is so delicate that differences in re- 
fractive index of .001 have been observed by the originator 
of the method. The extreme usefulness of the Becke 
method led the writer to endeavor to find a more detailed 
explanation of the phenomena observed. The construction, 
given below, it is believed, will satisfactorily explain all the 
phenomena. 
Let us assume A B, in fig. 1, to be a cross-section of 
two minerals with indices of 1.50 and 1.70 respectively, 
.whose plane of contact is represented by the line C D and is 
perpendicular to the page. Then if the section be in a mi- 
croscope arranged to give convergent polarized light the 
convergent rays will come from below and the objective lie 
above the cross-section as shown. Ray 1 is refracted so as 
*ivber die Bestimntbarkeil der Gesteinsgemengthelle, besonders der 
plagioklase auf Grand ihtes LichtbrechimgsvermSgens. Sitzungsb. k. Akad. 
Wiss. Wi.n. Mathem naturw. Classe. Bd. oil. Abtli. I. Juli, L893, pp. 358- 
376. Sep also Rosenbusch-lddings, a.Dpendix. 
(■ A. J. S., 4th Ser., vol. x, pp. 260-265, 1900. 
