Bauhaus-Universität Weimar

Optical Projection: A Treatise of the Use of the Lantern in Exhibition and Scientific Demonstration
Wright, Lewis
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, 
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.


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