1880.] Microscopy. 827 
mentioned the whole surface of each crystal is in focus at once, 
showing that the different sets of ribs are in the same plane, 
Where one crystal lies upon another, when examined under a 
power of a hundred and fifty diameters, both are in focus at once, 
showing that they are exceedingly thin and lie perfectly flat. 
. 
Sci. and A 
ANGULAR APERTURE.—Dr. Geo. E. Blackham’s paper on this 
subject, read at the Microscopical Congress at Indianapolis, has 
been published by the Industrial Publication Company of New 
York. The paper presents a comprehensive review, in a popular 
rather than a mathematical style, of the subject of the angular 
aperture of microscopic objectives. It is neatly published in an 
tions are used, 
and the only cement which I have thus far found to be tolerably 
