492 Scientific Proceedings, Royal Dublin Society. 
so. The same experiment may be made with cleavage plates of 
mica. 
It will be evident that in this method of examination colour 
effects due to rotary polarization, as in the case of quartz, are elimi- 
nated, for the rays evidently traverse the plate on their second 
passage, in the opposite direction as regards right and left to that 
which obtained on their first passage. However, as in thin sections 
these effects are in any case inconspicuous or inappreciable, this fact 
appears to be of no value in diagnosis. 
- Colour effects due to absorption are increased in intensity by 
the double transmission of the rays through the plate. 
The mode of procedure in the examination of a rock-section by 
the arrangement I have described is very simple. The illuminator, 
with its attached nicol, is first inserted above the objective, and the 
reflecting prism of the illuminator turned till white light, received 
from the clouds or from an artificial source of white light, entering 
the nicol of the illuminator, appears reflected to the eye from the little 
mirror which is laid upon the upper surface of the Abbe condenser. 
This adjustment is easily made. The analyser or upper nicol of 
the microscope is now placed in the position which it occupies 
when its plane of polarization is at 90° with the sub-stage nicol, 
and the nicol attached to the illuminator turned till the light 
reflected from the mirror is extinguished. The sub-stage polarizer 
and the nicol of the illuminator are now similarly oriented with 
regard to the analyser, and neither need be disturbed during sub- 
sequent operations. 
Removing the mirror and placing the rock-section on the 
microscope stage, examination for a maximum extinction tint is 
made by trial of several sections of the mineral being investigated 
in the usual manner. One of the brightest being selected and 
placed at 45° with its position of extinction and its tint then care- 
fully observed, the Abbe condenser is lowered and the little mirror 
placed on its upper surface by means of a forceps, and the Abbe 
again raised till the mirror is in contact with the slide, which, if © 
the crystal is small, should be face downwards on the stage. [The 
smallest crystals should be examined in this position without a 
cover-glass.]| The light transmitted from beneath is now cut off 
from the section, and that entering from the illuminator caught 
by the mirror and returned through the crystal to the eye. The 
