502 
8UMMARY OF CURRENT RESEARCHES RELATING TO 
observer an easy position for examining objects in liquids, and the means 
of measuring the angles of crystals. 
The angles of crystals are measured by cross-wires in the focus of 
the eye-lens, a divided circle attached to the body of the instrument, and 
a Vernier attached to the side of the brass work, carrying the rect- 
angular prism, which is adjusted by three screws, so that the hypothe- 
nuse is exactly at an angle of 45° with the optic axis. The adjustment 
is made by placing on the stage a slip of glass, ruled with fine cross- 
lines, which are made to coincide with the cross-wires in the eye- 
piece. 
The divided circle can be turned by rotating the tube at right angles 
Fig. 51. 
to the optic axis. The magnified image of the crystal also rotates, the 
angles being measured by the coincidence of the sides of the crystal with 
the cross- wires in the eye-piece. 
This arrangement is also useful in observing phases in polarization, 
the tube carrying the polarizing prism on the substage being rotated by 
clockwork, and four pins making electrical contact and ringing a bell, 
