| Absolute rate of growth | = 20 × ·025 µ = 0·5 µ per second |
| or | = 20 × 10-5 inch " |
| or | = 1⁄50,000 " " |
If we take the wave length of sodium light λ as our standard, the growth in length per second is equal to λ. This will give us some idea of the sensitiveness of the Crescograph employed in recording the movement of growth.
GROWTH-SCALE.
The Balanced Crescograph enables us not merely to determine the absolute rate of growth, but the slightest fluctuation in that rate.
Indicator Scale.—All necessity of calculation is obviated by the scale provided with the apparatus. The speed of clockwork which brings about the balance of growth is determined by the position of the adjusting screw S, the gradual lowering of which produces a continuous diminution of speed. A particular position of the screw therefore 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 corresponding 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 µ to 1·2 µ 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 D1 (= 0·5896 µ) and D2 (= 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 µ 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 Crescograph. In reality the problem before us is more intricate than simple measurement of change of 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 million times. In this method a very delicate astatic system of magnets undergoes deflection by the movement of a magnetised lever in its neighbourhood. A spot of light reflected from a small mirror attached to the astatic system, thus gives the highly magnified movement of the rate of growth, which may easily be raised to ten million times. I shall in the following describe the results obtained with this easily managed magnification of ten million times.
Determination of sensitiveness: Experiment 99.—A seedling of Zea Mays was placed on the Crescographic Balance; and the magnetic amplification, as stated above, was ten million times. With 18 strokes of the bell per minute the spot of light had a drift of + 266 cm. per minute to the right; this is because the growth was underbalanced. With faster rate of clock movement, i.e., 21 strokes in 68 seconds or 18·53 strokes per minute, the drift of the spot of light, owing to overbalance, was to the left at the rate of - 530 cm. per minute. Thus