490 Scientific Proceedings, Royal Dublin Society. 
condenser, when it may be lowered till flush with the surface of 
the stage. The polarized ray after passing downwards through 
the crystal under examination is reflected upwards by this mirror 
and passes a second time through the section before reaching the 
eye of the observer. If the crystal plate is placed with its prin- 
cipal axis at 45° with the plane of polarization of the ray incident 
from the illuminator, and, if higher, in the microscope, or above 
the eye-piece, is placed a second nicol set to extinction with the 
beam from the uncovered mirror, the interference tint received 
from the crystal is the maximum one proper toa plate of twice the 
actual thickness of the section. 
CAA 2 
KS 
For the mirror a dise of polished speculum metal, or silver (about 
one centimetre in diameter or less), gives the best results, and in 
the case of small crystals itis well to place the section cover-glass 
downwards upon the mirror, so that the reflecting surface is as close 
