396 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
back and with it the bolt, by the pin s coming in contact with the small 
projecting plate on the bolt. The two hemispherical lenses are held 
between the funnel-shaped depressions of bolt and axis. By pressure 
of the spiral spring the bolt is kept centered with respect to the axis, 
while it can move freely at 
both ends so that it rotates 
with the axis when the latter 
is turned. 
The goniometer for micro- 
scopic crystals (fig. 39) con- 
sists of a base-plate, on which 
is an upright carrying the 
divided circle T movable 
about a horizontal axis. The 
angle arm w ends in a ring 
which is parallel to the axis 
of the divided circle, and 
supports a second ring v 
movable in it. The hemi- 
sphere &, movable with friction in the ring v, has a conical opening 
bored through it, with the narrow central aperture in the flat surface. 
Along a radial groove runs the needle, at the point of which the crystal 
to be measured is fixed. It is kept in position by a spring, and is 
rotated by the milled head S. 
Some Improvements in the Crystallization Microscope.* — Prof. 0. 
Lehmann remarks that the old form of crystallization Microscope, 
described in Zeitschr. f. Instrumentenk., 1886, p. 325, suffers from the 
disadvantage that it is impossible to observe the preparation between 
crossed nicols during the heating. The method which first suggests 
Fig. 40. 
Fig. 39. 
itself for obviating this difficulty, that of placing the polarizing nicol be- 
fore the mirror, is unsatisfactory, owing to the large size of the nicol 
required. 
Fig. 40 represents the arrangement proposed by the firm of Zeiss, 
in which the polarizing nicol is replaced by two piles of glass plates. 
* Zeitschr. f. Instrumentenk., x. (1890) pp. 202-7. 
