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 D_{1} ( = 0·5896 µ) and D_{2} ( = 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 (1) 18 strokes per minute caused a drift of +266 cm. per minute. (2) 18·53 strokes per minute caused a drift of -530 cm. per minute. Hence by interpolation the exact balance is found to correspond to 18·177 strokes per minute. Therefore the absolute rate of growth = 18·177 × 0·025 µ per second. = 0·45 µ per second. = 0·000018 inch per second. We learn further from (1) and (2) that a variation of (18·53 - 18)/18·177 produces a change of drift of the spot of light from +266 to -530 cm., _i.e._, of 796 cm. per minute. As it is easy to detect a drift of 1 cm. per minute a variation of 0·53/(18·177 × 796), or 1 part in 27,000 may thus be detected by the Method of Balance. The spectroscopic method enabled us, as we saw, to detect change of wave length 1 part in a thousand. The sensibility of the Balanced Crescograph is thus seen to rival, if not surpass that of the spectroscope.