regularity of growth under constant conditions, but also the precision of the apparatus. It also shows that by keeping the external condition constant, the normal growth-rate could be maintained uniform for at least fifteen minutes. The magnified rate of growth is nearly 1 cm per second, and since it is quite easy to measure 0*5 mm., the Crescograph enables us to magnify and record a length of 0-0005 mm., that is to say, the sixteenth part of a wave of red light. The absolute rate of growth, moreover, can be determined in a period as short as 0*05 of a second. These facts will give some idea of the great possibilities of the Cresco- graph for future investigations.
As the period of experiment is very greatly shortened by the method of high magnification, 1 shall, in the determination of the absolute rate of growth, adopt a second as the unit of time, and /a, or tnicron, as the unit of length, — the micron, being a millionth part of a metre or a thousandth part of a millimeter.
If m be the magnifying power of the compound lever and I, the average distance between successive dots in mm. at intervals of t seconds then : —
the rate of growth = ^ x 10 ^/a per second. In the record given I = 9'5 mm. m = 10,000. ^ = 1 second.
Hence the rate of growth = -^^^ X 10 V per sec.
= 0.95/i per sec.
Having demonstrated the extreme sensitiveness and reliability of the apparatus, in quantitative determination, I shall next proceed to show its wide applicability for various researches relating to the influence of external agencies in modification of growth. For this two different methods are employed. In the first of these methods, the records are taken on a stationary plate : of these the
