December, 1910. 



KNOWLEDGE. 



501 



Burch and .\llen, show that colour liHndness causes 

 an abnormal increase of the persistence of the 

 primar\' colour or colours to which the subject 

 hajjpens to be blind. 



The same apparatus can thus be used as a test 

 of colour blindness. After we have obtained sufficient 

 statistics of the persistence of persons with normal 

 colour vision, we ma\" safeh- reckon an\- abnormal 



^^'hen a nervous impulse passes through a neural 

 track in the brain it reduces the resistance of the 

 track, so that following impulses pass with much 

 greater facility. 



This diminution of resistance by the passage of a 

 nervous impulse is called the law of neural habit. 

 According to James, the law of neural habit is the 

 foundation of all mental growth. From the psycho- 

 logical point of view this is equivalent 

 to saying that all mental growth con- 

 sists in the formation of associations 

 between different perceptions. 



In order to form an association 

 between two perceptions the neural 

 svstems in the cortex of the brain, 

 which represent these perceptions, 

 must be excited in immediate succes- 

 sion. For example, a child, when 

 coking at a ship, is told its name. 



Figure 1. Apparatus for Measuring Perseveration. 



deviation from this average as an indication of colour 

 blindness. 



The Determination of Mental Character. 



This persistence of a colour sensation after the 

 stimulus has stopped, which we have been discussing, 

 is identical, or verv closely related to a quality of 

 mind which the psychologists call Perseveration. 

 The amount of this Perseveration is probably a 

 fundamental propertv of the germ plasm, and. 

 therefore, inherited from our ancestors. 



The growth of the mental character of an 

 individual as he passes from childhood to maturity is 

 greatly influenced bv the amount of his Perseveration. 



Perseveration influences mental growth b\- the 

 control which it exercises upon the formation of 

 associations between two or more neural svstems. 



The name is ever afterwards associated with the 

 appearance of the ship in the mind of the child. 



^^'hat has taken place in the brain of the child ? 

 The neural system, a, representing the visual per- 

 ception of the ship, was first excited, and immediately 

 afterwards the neural system, b, representing the 

 auditorv perception of the name of the ship, was 

 excited. The effect of the total operation was to form 

 a path of reduced resistance between a and b, so that 

 when a is next excited a nervous current passes across 

 to b. and creates an image of the name of the ship. 



According to McDougall's theor}-, the excitement 

 of b exerts a guiding influence upon the discharge 

 from a. determining the discharge to take place 

 from a to b. A physical analogy which helps to 

 explain this phenomenon is the passage of an electric 

 spark from a point at high potential to another point 



