164 PHYSIOLOGY OF MUSCLES AND NERVES. 



escent position, and its reflection is observed through 

 the lens as described in Chap. IV., § 3. This may also 

 be nsed to render the deflection visible to a large audi- 

 ence,- by allowing the light of a sufficiently powerful 

 lamp to fall on the mirror and throwing the reflection 

 on to a screen by means of a lens. In order to in- 

 crease the sensitiveness of the instrument, the influence 

 of gravitation on the deflecting magnet is decreased, as 

 already described, 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 the animal substances which are to be 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 present or not. But the 

 more sensitive is the multiplier, the harder is it to 

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

 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 coniiected 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 deflection 

 of the needle always occurs, owing to the fact that the 

 copper wires 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 cleaning; but this 

 homogeneity soon disappears, so that even with this 

 metal currents result which depend solely on the dis- 

 similar nature of the metallic surfaces. Fortunately, 



