394 SUMMARY OP CURRENT RESEARCHES RELATING TO 
illumination of tlie preparation when the lower objectives are used. 
This lens forms the lower member of the condensing system, which 
consists of three lenses, the other two being connected together but 
detached from the polarizer. The common socket of the two upper 
lenses is supported in a ring which forms the end of the arm b of a 
rotating plate fitted in the object-stage. By means of a weak spiral 
spring in the ring-holder, the socket follows the movement of the 
polarizer, so that the whole condensing system can be adjusted by the 
pinion which effects the movement of the polarizer. By a second arm b' 
from the rotating plate of the lens-holder, the upper pair of lenses can 
be moved to one side beneath the mechanical stage, so that the change 
from convergent to parallel light and vice versa can be rapidly effected 
without moving the preparation. 
The coarse-adjustment of the Microscope is by rack and pinion. The 
fine-adjustment screw has a pitch of 0*5 mm. The head is divided into 
100 parts, and a vernier reads to a fifth of a division, i. e. to 0‘ 001 mm. 
The end of the body-tube carrying the objective is movable by two fiue 
screws for centering. To facilitate the change of objectives, the latter 
are not screwed on, but held by the clamp k. Immediately above the 
clamp is a slit for the introduction of a Klein’s plate, quarter-wave 
plate, &c. The analysing nicol N is inserted in a wider opening at the 
lower end of the body-tube. Another opening K serves for the intro- 
duction of the auxiliary objective into the draw-tube R. The lens is 
fastened in the slide /, and forms with the Ramsden eye-piece a complete 
Microscope, with a magnification of about five times. This constant 
magnification is advantageous for measuring the apparent optic axial 
angle. The draw-tube carries a millimetre scale which gives the 
distance of the eye-piece from the objective. Among the accessories of 
the Microscope are the illuminating apparatus and spectropolarizer of 
Abbe, the twin-nicol for stauroscopic measurements, and the illuminating 
arrangement of Sorby which serves for the observation of the internal 
and external conical refraction. 
Of the special eye-pieces, the goniometer eye-piece consists of a 
Ramsden eye-piece which is directed upon cross wires centered by four 
adjusting screws exactly in the axis of a divided circle. 
The quartz-wedge comparator shown in fig. 37 is a modified form of 
that of Michel Levy. It slides in the tube T of the Microscope, and in 
this part consists of an ordinary weak eye-piece, in which, in the place 
of the usual diaphragm, is a double prism of glass P P', with the hypo- 
thenuse faces cemented together. The liypothenuse P' is silvered witli 
the exception of a small circle in the centre, through which the polariza- 
tion tint of the preparation is seen. The main part of the comparator is 
contained in the side tube. Rays from the mirror s are diverted at 
right angles by the prism r into the polarizer n , the rotation of which 
can be measured on a divided circle. The lens l concentrates the light 
upon the quartz wedge q, behind which is a diaphragm with a very small 
aperture. The quartz wedge is fastened in the slide S', which is moved 
by rack and pinion, and its position is given by the vernier Y. The 
light passes through the analyser n' to the lens o, and is reflected 
from the liypothenuse of the prism P' in the direction of the axis 
of the eye-piece. The polarization tint of the preparation is thus seen 
