234 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Its position is between the Nicol’s prism and the lower lens of the con- 
densing system which is attached to the polariser. For the observation 
of objects in light of definite wave-length, the Rollet spectropolariser as 
modified by Dippel and Abbe (fig. 30) is serviceable. It is connected 
with the Microscope by the insertion of the tube R in the socket of 
the polariser. The light reflected from the mirror s passes through 
the polarising nicol Po to the movable slit, and thence through the 
collimator C and the prism combination P P to the objective 0, which 
Fig. 28 . 
projects a spectrum upon the object to be examined. By reflection on a 
prism face, a scale of wave-length is projected, together with the spec- 
trum, in the plane of the image. The scale is illuminated by the mirror 
s pi. 
The screws s and s' serve to adjust the prisms so as to bring cor- 
responding positions of the spectrum and scale into coincidence. To 
bring definite parts of the spectrum in the middle of the field, the 
objective 0 can be displaced by the slide u. 
For the observation of minerals with wide axial angle Fuess prepares 
a special condenser and objective system (immersion), whose lenses are 
cut out of strongly refracting flint glass. 
Of axial angle apparatus which can be attached to the Microscope- 
stage there are several varieties. 
In the apparatus shown in fig. 31, for the measurement of the axial 
angle in very small plates, the principle of Adams is used ; in larger 
plates or crystals the usual medium of immersion is a strongly refractive 
liquid. 
