DETECTION OF LATENT STIMULUS 457 



Positive and negative after-effects. — We have seen that 

 when a moderately strong stimulus acts on a responding 

 organ, a short time elapses before the initiation of the re- 

 sponse, and this is known as the latent period ; but when 

 response has been initiated, it persists for some time, even on 

 the cessation of the stimulus, and this is known as the after- 

 effect. I shall, however, for important reasons, which will 

 appear later, further distinguish it as the positive after-effect. 

 By the term positive after-effect, then, is meant the continua- 

 tion of a response evoked by external stimulus, on the cessation 

 of that stimulus. We may, for example, imagine a heavy 

 elastic spring immersed in a viscous fluid. If this be sub- 

 jected to a sudden compressional blow, then, after a short 

 latent period, it will begin to undergo compression, and this 

 compressional movement will continue for some time, even 

 on the cessation of the blow that caused it, thus exhibiting a 

 positive after-effect. But a spring compressed in this manner 

 contains some amount of latent or potential energy, on 

 account of which it next begins to expand, exhibiting a 

 movement opposite to the first. This second movement, due 

 to the latent energy, we may distinguish as the negative after- 

 effect. This negative after-effect, it should further be stated, 

 may sometimes be separated from the direct effect by a con- 

 siderable interval of time. This may be seen in a viscous 

 wire subjected to a torsional impulse. After the twisting 

 has ceased, some time elapses before the wire is seen to 

 begin the contrary, or negative, movement of untwisting, 

 which is accomplished very slowly, and may even take hours 

 to complete. 



Now, it occurred to me that, in the response of growth, 

 it was possible to find a means of detecting whether the 

 external stimulus in the case of living tissue might, or might 

 not, become partially latent, to be similarly manifested later, 

 in the form of the negative after-effect ; for if we take a 

 balanced horizontal record of growth, then the direct effect 

 of external stimulus will be seen in that retardation which 

 is shown in the shifting of the line — here, for convenience of 



