236 OUTPOSTS OF THE INTELLIGENCE SERVICE 



(rapid adaptation), touch, tickle, etc., a single .stimulation giv^cs 

 rise to the same electrical variations — frequency, amplitude and 

 duration — irrespective of the intensity of the stimulus. A graded 

 response can be obtained only if several end -organs are stimulated 

 simultaneously. 



To offset this lack of discriminative power the phasic end-organs 

 have a very short refractory period, and so are able to accept and 

 transmit a subsequent stimulus in a fraction of a second. Per- 

 sistence of tickling, say by distortion of hairs, produces a rapid 

 series of electrical variations. Postural receptors are capable of 

 giving a graded response. We can recognise a pressure stimulus 

 produced by 1 gram as being less than that produced by 5 grams. 

 The movements of the mercury in the capillary electrometer 

 indicate that, in the former case, a very brief discharge has occurred, 

 while in the latter case, the duration of the discharge has been in- 

 creased. Amplitude [i.e. potential difference) and frequency {i.e. rate 

 of discharge) are the same for any weight giving rise to a sensation 

 of pure pressure. Similar findings from experiments on other end- 

 organs lead to the same conclusion, viz. the receptor when stimulated 

 causes an impulse to he propagated along a series of units containing 

 ions, so that each unit becomes first negatively and then positively 

 charged. The potential developed is a constant ; the rate at which 

 each cycle of electrical charges appears, varies from nerve to nerve 

 {between 5 and 100 cycles jjer second), but is characteristic for any 

 particular nerve fibre. The time during which the series of cycles 

 jjersists is a measure of the intensity of the stimulus where adaptation 

 is slow enough to allow of it. 



The organism is subject to stimulation from various forms of 

 energy which may be classified into vibratory and chemical. 



A. Vibratory Energy. 



1. Mechanical impacts are received by the tactile corpuscles of 

 the skin. They may be perceived as separate stimuli even when 

 they arrive as rapidly as 150 per second. 



2. Slow vibrations especially in air are received by the ear. The 

 himian ear may be stimulated by vibrations ranging from 16 to 

 40,000 per second. Practice may extend this range. 



3. Rapid vibrations in ether. 



(«) Radiant heat. Vibrations with a frequency of between 

 3 billions and 400 billions per second stimulate the temperature 

 receptors of the skin. 



(6) Light. The retina is capable of receiving as light, ether 

 waves, the frequency of which varies between about 400 billions 

 and 800 billions per second. 



