240
OPTICAL PROJECTION
on another part of the screen, and from the other system of a
demonstrating bi-unial lantern, the preparation itself, in order
to show the arrangements. The time will not, however, be far
distant when every medical school and college will possess an
adequate projecting microscope ; and with such an instrument
these feeble currents may be shown directly by Professor
M‘Kendrick’s modification of Lippmann’s capillary electro¬
meter, which can be easily constructed by any person at all
accustomed to scientific manipulation. A piece of narrow
and thin glass tube a b is taken, the ends bent up to form
small cups, and the middle drawn into a very fine capillary
bore. Immersing one end in mercury covered with dilute
sulphuric acid, this is so drawn into the tube that a very
minute portion of dilute acid is brought into the centre at c
between the two columns of mercury, and a platinum wire is
introduced into the mercury at
each end. To secure a sensitive
instrument, perfectly clean glass,
Q
fio. 127 —capillary Electrometer acid, and mercury are necessary ;
and the slightest air-bubble must
be avoided. The instrument is then carefully mounted on a
glass slip as a slide for the microscope, and it will be better
to lay over the capillary portion a piece of thin cover-glass,
and fill the space between, surrounding the tube, with Canada
balsam, which optically abolishes the glass tube, and enables
the thread of mercury with its break of acid to be sharply
focussed. Placing the slide on the stage of the microscope,
and connecting the wires with the nerve or muscle through
non-polarisable electrodes, after the manner of Du Bois-Rey-
mond or otherwise,1 the current will produce a movement,
1 Details of all these arrangements must be sought in some of the numerous
text-books devoted to such matters. I have taken several from Professor
M‘Gregor- Robert son’s Elements of Physiological Physics ; but it is no part of
my purpose to describe more than bears upon the projection of them, and
may make that portion of the subject intelligible to those whose special business
it is to deal with it.