338 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
and hold the ring B loosely in position, allowing it to be revolved by 
means of its milled flange, which projects at E. Within the ring B is 
screwed a brass disc F, which carries the upper, thin glass, and which 
Fig. 86. 
is attached by the screws G G. The screws G G and C fitting into 
lioles in the lower plate A, and the disc F respectively, prevent the 
disc from revolving; and, when the ring E is turned, the two thin 
glasses are moved toward or away from one another. 
The slides D D and the ring B, together with the disc F, are re- 
moved for arranging the object on the lower cover glass; and when 
replaced, by revolving the ring at E, any desired amount of compression 
may be obtained. The object having been arranged, either side may 
be examined with equal facility, as the compressor is reversible. 
When a very small object is to be examined, a small circular cover 
glass should be cemented with Canada balsam to the lower cover-glass, 
and the object is thus confined to the centre of the field. 
Fig. 87. Fig. 88. 
Beck’s Achromatic Condenser. — This piece of apparatus is designed 
as a well-corrected condenser suitable for the most delicate liigh-power 
investigation. It has an aperture of 1*00 N.A., or the maximum that 
