REFRACTIVE INDICES. 91 
58, bi", in this case the rays emerge in n\ t although actually bent toward tit, 
and if the microscope be focussed on the point D the maximum intensity 
will appear on the side of n\\ and a false inference might be drawn from the 
position of the Becke line relative to the junction line between the two 
minerals. As the objective is raised, the actual line would recede from the 
junction line on the side of n\ and one might easily conclude that n\ had the 
higher refractive index. Careful attention to these phenomena is sufficient, 
however, to obviate all such errors. The dotted lines in Fig. 58 indicate 
the paths of the rays which are transmitted just beyond the boundary line 
between n\ and n-t. 
Still another factor influences the appearance of the image, and is made 
use of in practical microscope work. The average thickness of a thin sec- 
tion of rock is about o . 02 mm. ; the equivalent optical length of any given 
plate is its thickness increased in the ratio of its refractive index (Fig. 59). 
FIG. 59. 
If, therefore, the objective be focussed sharply on the lower surface AB 
of one of the mineral sections of refractive index n\, the lower surface AC of 
an adjacent plate of different refractive index will not appear, as seen through 
the plate itself, to be in sharp focus and the objective must be raised or 
lowered to obtain a good image, with the result that certain of the mineral 
grains stand out in relief, while others appear to be below the general level, 
the rule being that the higher the refractive index of a plate the more pro- 
nounced its relief. As the surfaces of thin sections are not highly polished, 
the phenomena of irregular refraction and total reflection appear on the 
surfaces of grains whose refractive index is greatly different, either higher or 
lower, from that of the enveloping Canada balsam of the mount. Such 
mottled, pock-marked surfaces (shagreened surfaces) are characteristic of 
a great difference in relative refringence between the mineral plate and the 
surrounding Canada balsam.* The scattering of light under these condi- 
ditions is an important factor bearing on the development of the image by 
the optical system of the microscope. 
*The average refractive index of Canada balsam has recently been redetermined by W. Schaller and P. 
Calkins on several hundreds of thin sections of different sources and ages and found to average about 1.540 
(Amer. Journ. Sci. (4), 39,324. 1910) and to vary between 1.535 ad -545- The refractive index of the 
uncooked balsam is about 1.524; see also E. A. \Vulfing, Sitzber d. Hcidelberger Acad. d. Wissen. Math.- 
natur. Kl. 1911, 1-26. 
