REFRACTIVE INDICES. 95 
objective noticeably and only marginal rays are used. This principle, 
which is familiar to biologists as that which underlies dark-ground illumina- 
tion in which both reflection and refraction are essential factors, is rarely 
used by petrologists, but is required whenever high powers are employed. 
Two methods* have been suggested for this purpose: (a) by the use of a 
condenser of wide aperture in which the central rays are stopped out 
(hollow-cone illumination), or, (b) by using a narrow beam of light from the 
condenser and a small stop in the eye-circle of the ocular, as first suggested 
by S. Exnerf in 1 885 in his microrefractometer, which in turn has been 
applied by F. BeckeJ to microscopic work. 
The present writer has also had constructed the small device shown in 
Plate i, Fig. 2, which fits above the ocular and carries a rotating plate c, 
into which small disks of cover-glass have been inserted ; on these in turn 
small, thin, opaque brass disks of different sizes (0.5 to 3 mm. diameter) 
are cemented and serve as central stops when placed in the eye-circle of the 
ocular. With this device both dark-ground illumination and oblique 
illumination as well can be satisfactorily obtained. In the examination of 
very minute particles fairly high powers are frequently necessary and this 
method of oblique illumination is then the only feasible one not involving 
complicated apparatus. In applying the method care should be taken to 
place the small opaque disks precisely in the plane of the Ramsden disk 
above the ocular. As a general rule, it is unnecessary to use high powers 
and the above device is rarely required. 
THE BECKE LINE METHOD. 
In principle this method, || which is described in the standard text-books, 
does not differ essentially from that described by Maschke. As noted 
above, a narrow cone of incident light is required and the concentration of 
light along the margin of the higher refracting of two adjacent mineral 
plates is used to determine their relative refringence. On raising or lowering 
the objective, the intense band of white light (Becke line) is observed toward 
or away from the margin, the rule being that on raising the objective the 
Becke line moves toward the mineral of the higher refractive index. High- 
power objectives are ordinarily used in tests with the Becke line, because of 
their higher numerical aperture and consequent sharper resolution in depth. 
If the aperture of the substage condenser be not sufficiently decreased, the 
light is concentrated on both sides of the boundary plane ; there are appar- 
ently two Becke lines present, moving in opposite directions, and one is at a 
loss to determine which is the correct one. The remedy is obviously to 
reduce the aperture of the cone of illumination either by lowering the con- 
denser or by closing the iris diafram, or by adopting both methods. (See 
page 53-) 
*For detailed descriptions of these methods and of the principles underlying them, see Sir A. Wright. 
Principles of Microscopy, London, 1906. 
tArchiv. f. Mikroskop. Anat., 25, 97, 1885. 
IS. W. A. 103, 358-376. >93; T. M. P. M.. 13, 385-388. 1893. 
||P. Becke. S. W. A., I. Abt.. 102. 358. 1892; T. M. P. M.. 13, 385. 1893. Compare also the recent 
paper by G. W. Grabham (Miner. Mag., 15, 335-349. 1910). in which a detailed discussion of the Becke 
line method is given and certain features are emphasized which are not ordinarily taken into considera- 
tion, especially the effects produced by obliquely incident rays and by inclined junction planes. See also 
J. L. C. Schroeder van der Kotk. Zeitschr. Wissen. Mikrosk. 8, 456-458, 1891. 
