600 COMPARATIVE ELECTRO-PHYSIOLOGY 



sensitive substance may be made the fourth arm of the 

 Wheatstone's bridge, and the responsive galvanometric de- 

 flection and subsequent recovery of the spot of light by 

 the upsetting of the balance, under the action of stimulus 

 of radiation is recorded in the usual manner, on a moving 

 photographic plate. Or the sensitive substance may be 

 placed in series with a galvanometer, a small E.M.F. giving 

 a steady permanent deflection, Taking first selenium as the 

 sensitive substance, the molecular change induced by the 

 action of light, with its concomitant variation of resistance, 

 causes a deflection of the galvanometer spot of light On 

 the cessation of the stimulus, molecular recovery takes place, 

 and the deflected spot of light returns to its original position. 

 A series of such responses will be found on referring to 

 page 3, fig. 3. The parallel method employed in recording 



the responsive resistivity varia- 

 tion of masses of metallic par- 

 ticles of various kinds, under 

 the stimulus of electric radia- 

 tion, will be understood from 



FIG. 367. Method of Resistivity && &?' On obtaining records 

 Variation of the responses given under 



Sensitive metallic particles placed in this method, I find, as I pointed 

 tube in series with galvanometer . A . ,, 



and E.M.F. This gives a steady out in the first chapter, that 

 permanent deflection. Stimulus t h e responding substances are 



of electric radiation induces a 



responsive variation of resistance of two different types. The 



of . f 



be taken as the example, re- 



spond by diminution, or negative variation of resistance. The 



second, illustrated by potassium or arsenic, respond by an in- 



crease, or positive variation of resistance. In living tissues 



also, tested by various modes of response, we have seen two 



opposite types to occur highly excitable nerve giving one, 



say, negative, while skin, on the other hand, gave the positive. 



In the case of the inorganic substances referred to, we 



have extreme types, whose response is generally either 



.positive or negative. There are, however, intermediate 



