104. PHYSIOLOGY OF Ml- LES .\M> NKKVF.S. 



escenl portion, ami Its reflection is observed through 

 the leu- afl de-erlbed in Chap. IV., 3. This m:iy :ilso 

 he used to render the deflection visible to a large audi- 

 ence, l>y allowing tin- light of a sufficiently powerful 

 l.-inij) io full on the mirror and throwing tlic reflection 

 on to a screen by means of a l-'iis. In order to in- 

 crease t In- sensit i\ eness of the inst riiiin-iit, the influence 

 of gravitation on the deflecting magnet is decreased, as 

 already de>cribed, by means of a properly arranged 

 magnet. 



5. Having, in one or other of these ways, provided 

 as sensitive a multiplier as may be, all that is necessary 

 is to connect (lie animal substances which are to l>e ex- 

 amined with this, and then to observe whether deflec- 

 tion occurs or not. ; whether, that is, with the arrange- 

 ment selected a current is proent or not. ]>ut the 

 more sensitive is the multiplier, the harder is it to 

 connect any part of an animal with it in such a uay 

 that no current occurs, and it would be a mistake to 

 suppose that all these currents are elicited by the ani- 

 mal substances themselves. If, for example, the ends 

 of the wires of the multiplier are connected with two 

 wires of the same metal for example, copper; and if 

 these wires are immersed in a conducting fluid for 

 example, diluted sulphuric acid considerable deflect ion 

 of the needle a 1 \\ays occurs, o\\ing to the fact that the 

 copper \\ ires are never so homogeneous that they do 

 not themselves generate a slight current. If platinum 

 wires are used instead of copper, these can, it is true, 

 be rendered homogeneous by careful denning; but this 

 homogeneity soon di-app--ar<. >o that even with this 

 metal cnrn nt- result \shidi depend solely on the dis- 

 similar nal lire of the metallic surfaces. Fortunately, 



