62 THE AMERICAN MONTHLY [Mar 
as it only measured 180° inair. The second apertometer 
was merely amm.scale placed in the turn-out ring of the 
condenser. Thecondenser and an objective of known ap- 
erture were focussed on the same plane, and the N. A. 
value of a division in the mm. scale was found by an ex- 
amination of the back lens of the objective. When any 
other objective was employed, the number of divisions 
visible at the back lens, multiplied by the determined 
value of one division, gave the aperture of the objective. 
Mr. Angus showed the value of such a quick method of 
measuring aperture by the ease with which the size of 
diaphragms and stops could be found; he also gave pro- 
portions of stops to apertures which furnished the best 
dark-ground effects. Stops were shown that were made 
of black paper gummed to glass disks, cut to fit the con- 
denser turn-out ring. The club’s best thanks were ac- 
corded to Mr. Angus for his explanations which, it was 
hoped, could be put into a form suitable for publication 
in the journal. 
A sliding microtome by Reichert was exhibited by Mr. 
C. Lees Curties, and described by the Secretary. This 
machine was on the “Thoma” model, with mechanical ad- 
justments to every part, even to the tilting of the knife. 
The micrometer screw was reversible, thus obviating te- 
dious unwinding. The instrument was very moderate in 
price. Mr. D. J. Scourfield, F. R. M. S., read a Paper on 
“The Ephippia of the Lyceid Entromostraca.” The au- 
thor originally intended to wait for sufficient material for 
a more comprehensive paper, but in view of the wideness 
of the subject preferred to deal with a part which had 
been fairly well worked at. He described the principal 
features of the formation and casting of these particular 
ephippia ; the curious membrane around the egg, the thick- 
ening of the posterior margins of the valves, the growth 
of the line of cells across the valve, where the separation 
eventually takes place, and other apparently constant ac- 
