34 THE SIGNS OF LIFE [LECT. 



to above as the "parting bow." If during the exposure to light 

 there is a tug-of-war between positive and negative current, 

 with predominance of the former, and if at the end of exposure 

 both currents should cease, but the negative cease more rapidly 

 than the positive, then we should witness what actually does 

 happen, viz., a short movement in the positive direction preceding 

 the return to a state of rest. And finally, on turning back to the 

 nicer examination of the rising effect at the outset of exposure, 

 we find another sign of an opposition between two contrary and 

 all but synchronously developing currents. There is often at this 

 point a false latent period, or period of hesitation, perceptible on 

 simple observation, or, better still, by means of records where 

 the beginning of an exposure has been mechanically signalled, 

 amounting to several seconds, and intelligible only on the sup- 

 position that our galvanometric magnet is, so to say, trembling 

 in the balance between two opposite and almost perfectly con- 

 gruent forces. And generally, indeed, on the record of such a 

 false latent period, we may detect signs of such a preliminary 

 struggle, as if an initial positive movement had been forthwith 

 interfered with for a brief period by a contrary negative move- 

 ment. On some records the magnet remains almost perfectly at 

 rest during several seconds of hesitation ; on others it is caught 

 back by a sharp negative jerk ; on others still the curve of positive 

 movement is only slightly hitched or notched in its progressive 

 development. All these facts viz., the transitional responses 

 from positive to negative, the immediate conversion from positive 

 to negative by compression, the initial period of hesitation, the 

 terminal "parting bow" point to one and the same conclusion, 

 viz., that the retina, when exposed to light, is the seat of two 

 contrary electro-motive changes. And it matters little whether 

 you imagine two single processes or one double process behind 

 the movements of the machinery. 



Thus, following a path step by step as it happens to lead us, 

 we find ourselves quite unexpectedly at a place where theory 

 and doctrine seem to be quite familiar to us. We are all of us 

 more or less intimately acquainted with Hering's theory of 

 colour-vision setting forth that contrary processes of dissimila- 

 tion and assimilation are aroused by complementary colours, 



