262 LIFE MOVEMENTS IN PLANTS 



gradual lowering of which produces a continuous diminu- 

 tion of ppeed. A particular position of the screw thei-efore 

 indicates a definite rate of subsidence for balancing growth. 

 By a simple mechanism the up or down movement of the 

 screw causes rotation of an index pivoted at the centre of 

 a circular scale. Each division of the scale is calibrated 

 by counting the cori-psponding number of strokes of the 

 bell per minute at different positions of the adjusting screw. 

 The scale is calibrated in this manner to indicate different 

 rates of growth from 0"2 fjL to 1*2 fx per second. 



The determination of the rate of growth now becomes 

 extremely simple. Few turns of the screw bring about 

 the balance of growth and the resulting position of the 

 index against the circular scale automatically indicates the 

 absolute rate. The procedure is even simpler and more 

 expeditious than the determination of the weight of a 

 substance by means of a balance. 



SENSITIVENESS OF THE CRESCOGRAPHIC BALANCE. 



Perhaps the most delicate method of measuring lengths 

 is that afforded indirectly by the spectrum of a light. A 

 good spectroscope resolves differences of wave lengths of Dj 

 (=0-5896 /x) and D.. (=0-5890) i.e. of 1 part in a thousand. 

 The average rate of growth of Zea Mays is of this order ; 

 being about 0-5 yu. per second. Let us consider the question 

 of the possibility of detecting a fractional variation of the 

 ultra-microscopic length by means of the Balanced Cresco- 

 graph. In reality the problem before us is more intricate 

 than simple measurement of change ui length ; for we have 

 to determine the rate of variation of length. 



The sensitiveness of the balance will, it is obvious, 

 depend on the magnifying power of the Crescograph. By 

 the Method of Magnetic Amplification referred to in page 

 170, I have succeeded in obtaining a magnification of ten 



