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.