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 & 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, 



FIQ. i27.-capiiiai7 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-Kobertson'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 

 ft is to deal with it. 



