THE RECORD OF GROWTH-RESPONSE 413 



that time. A variation of growth will produce a variation 

 in the slope of the curve. When this variation of growth, 

 however, is slight, the variation of slope of the curve is so 

 small as not to be detected. It was in order to detect and 

 measure such small variations that I devised the Method of 

 Balance, in which the average rate of growth is represented 

 by the horizontal line of balance, any fluctuation, even the 

 slightest, appearing as a deviation from this horizontal. The 

 influence of various agencies, again, may be displayed in a 

 marked manner, by using this method ; for in such cases we 

 do not so much require the rate of growth itself, as the 

 variation — i.e. acceleration or retardation- in the normal 

 rate, which is induced by one agent or another. 



The principle of the Method of Balance consists in 

 making the spot of light — which is moving in response to 

 growth — become stationary', by subjecting it to a compen- 

 sating movement. An example will make this clear. We 

 shall suppose the average rate of growth to be V2 mm. 

 per hour. This will cause an excursion of the moving spot 

 of light, from, say, left to right, through 1,200 mm. by the 

 end of the hour, in that case v/here the magnification is 1,000. 

 Had the growth been uniform, this would have meant a 

 movement of 20 mm. per minute. But if not uniform, the 

 rate might sometimes have risen above, and at others fallen 

 below, this average. If now we subject the spot of light to a 

 uniform compensating movement, such as by itself would 

 have made it move from right to left of the recording sur- 

 face, to the extent of 1,200 mm. by the end of the hour, we 

 shall find that, being acted on by these two opposite move- 

 ments, of growth and compensation, the spot will remain 

 approximately on a single base line of compensation. The 

 fluctuations, or variations, which have occurred in this average 

 rate of growth, will, however, be recorded as deviations 

 to one side or other of this mean neutral line. Thus it 

 will be seen that the slightest deviation from a uniform rate 

 of growth, will be found displayed by the record of the 

 moving spot of light. We are further enabled, from our 



