378 MICROSCOPY. 
mind is charmed with the simplicity of the means by which these 
motions are controlled. In the cell here described, no foreign 
liquid is added to the material under examination. Moreover, if 
each cell be entirely filled, but with liquids of different densities, 
the cell holding the denser liquid being placed slightly uppermost 
upon the rotating stage of the microscope, the action of gravity 
will cause two currents to flow in opposite directions through the 
communicating channels, and in this way the phenomena of trans- 
fusion, crystallization, etc., may be observed for a considerable 
length of time, which otherwise are brought to sight only with 
difficulty. At the conclusion of the description, the ingenious and 
useful device was highly praised by those members present, who 
were best able to appreciate its value, and its exhibition beneath 
the microscope was the occasion of much interest. 
AERIAL Stace Micrometers.—Dr. Pigott has called the atten- 
tion of the Royal Microscopical Society to a novel mode of using 
micrometers. He places the micrometer below the achromatic 
condenser, and thus employs its image as a stage micrometer, fo- 
cussing the condenser so as to make the image of the micrometer 
coincide with the plane of the object on the stage. This remedies 
the greatest defect of other stage micrometers (as Fraunhofer’s), 
since the accuracy which is necessarily diminished in proportion 
to the magnifying power employed, is at the same time increased 
by the whole amplifying power of the achromatic condenser. 
Hence this arrangement more nearly resembles in accuracy the 
ocular micrometers, and it might with nearly equal propriety 
be called an eye-piece micrometer, since its second image is formed 
in the ocular along with that of the object. It possesses the valu- 
able property of reading off the size of objects directly, without 
troublesome computation and without allowance for the power 
of the ocular. Either the cobweb micrometer or the lines ruled 
on glass may be used, and the arrangement should be such that 
the micrometer lines should gaei on the stage in precisely a defi- 
nite proportion of their natural s An accuracy of 470000 of 
an inch is theoretically quite ssihsdatte by this plan. With the 
cobweb micrometer this arrangement seems nearly faultless, save 
the first trouble of combining the apparatus so as to get a perfectly 
accurate reading : but, with lines on glass, the glass plate, with its 
imperfections as well as its lines, necessarily gives an image W which 
