Page:Collected Physical Papers.djvu/378

358 that this method of super-magnification would be of great help in many physical investigations.

Such an enormous magnification can not be employed in ordinary investigations on growth, for the indicating spot of light rushes on like a flash. I had, therefore, to reduce the magnification to a million times by which demonstrations of a striking character on various phenomena of growth can be given before a large audience. The following may be taken as a typical example:—

The normal rate of growth of the experimental flower-bud of Crinum Lily was 0.0006 mm. per second. A scale 3 metres long divided into centimetres was placed against the screen. A metronome beating half seconds is started at the moment when the spot of light transits across the zero division; the number of beats is counted till the index traverses 300 cm. At the normal temperature of the room (30° C.) the index traversed 300 cm. in 5 seconds. As it is easy to measure 1 mm. on the scale, the Magnetic Crescograph enables measurement of growth in a period shorter than 1/500th of a second.

After observing the normal rate at the temperature of the room, the plant chamber was cooled to 26° C. by blowing in cooled water vapour; the time now taken by the spot of light to traverse the scale was 20 seconds, i.e., the growth-rate was depressed to one-fourth. Under continuous lowering of temperature there was a continuous retardation, till at 21° C. growth was arrested. Warm vapour was next introduced raising the temperature of the plant-chamber to 35° C. The