BEALE, ON THE MICROSCOPE, 45 
the stage, and all other movements and adjustments, should be smooth and 
even, without any tendency to a jerking or irregular action. 
“ Necessary Apparatus.—Kvery student’s microscope should be provided 
with a neutral tint glass reflector for drawing and measuring objects, a 
diaphragm, to the under part of which is fitted a tube to receive an achro- 
matic condenser, or polarizing apparatus, a bull’s eye condenser, one shallow 
eye-peice, and two powers—a low one, magnifying from 20 to 40 diameters, 
and a quarter of an inch which magnifies at least 180 diameters. 
“These instruments should be conveniently packed in the case with the 
microscope. The polarizing apparatus and the achromatic condenser can be 
purchased afterwards. ‘lhe cost of the microscope without these instru- 
ments, but including the other apparatus mentioned, should not be more 
than from six to seven pounds ; and if the microscope be mounted upon a 
cast-iron foot instead of a brass one, it may be obtained for about a pound 
less, without its practical utility being in any way impaired.” 
The second lecture treats of illumination, of reflected light, 
transmitted light, and polarized light; also of artificial light 
and the drawing of microscopical specimens, and the mea- 
suring of objects. We give one extract on an important 
practical point, and which some otherwise competent obser- 
vers do not appear to have mastered : 
“* On Ascertaining the Magnifying Power of Object-glasses.—Permit me now 
to say a few words upon the magnifying power of the different lenses. 
Although the several object-glasses are termed one inch, one quarter of an 
inch, one eighth, &c., the magnifying power of each is not definite, and the 
quarters of some makers magnify many times more than those of others. 
It is well, therefore, that every observer should be able to ascertain for him- 
self the magnifying power of his different glasses. Suppose I wish to know 
how much this French quarter magnifies. The one-thousandth of an inch 
micrometer is placed in the field, and the magnified image is thrown by 
means of the neutral tint glass reflector upon this scale, divided into inches 
and tenths of inches. The magnified one-thousandth of an inch covers about 
two-tenths of an inch, and consequently this glass magnifies about 200 
diameters; for if it covered one inch, the thousandth of an inch must have been 
magnified 1,000 times, but in this case it only corresponds to the one-fifth 
of an inch, and therefore the one-thousandth is magnified 200 times. For 
lower powers the one-hundredth of an inch scale may be employed. The 
manner of ascertaining the magnifying power is therefore exceedingly sim- 
ple; but it is very important for the observer to know the magnifying 
power of every lens, and he should ascertain this before he commences te 
make any observations.” 
The third lecture is devoted to the instruments required in 
addition to the microscope in various minute investigations. 
Scissors, balsams, cements, glasses, preservative solutions, 
and other things are treated of in this lecture. The fourth 
lecture is devoted to instructions for making cells for micro- 
scopical preparations. This is a very important art, and will 
save a good deal of expense to those who keep collections of 
specimens. We would, however, warn those who have but 
little time to spare against being too anxious to make every- 
