Page:Collected Physical Papers.djvu/367

 XXVI

THE HIGH MAGNIFICATION CRESCOGRAPH

The difficulty of investigations on growth arises from its extreme slowness, which is two thousand times slower than the movement of the snail. The auxanometers usually employed magnifies growth to about 20 times; under this magnification several hours must elapse before growth becomes perceptible. During this long period the external conditions, such as light and warmth, undergo change thereby confusing and vitiating the result. The external conditions can be kept constant for a few minutes only; hence the effect of variation of an individual factor can only be found by increasing the magnification to about ten thousand times and thus reducing the period of the experiment. The apparatus devised for this purpose not only produces this enormous magnification but also automatically records the rate of growth and its induced variation in the course of time as short as a minute or so.

The recorder consists of a compound system of two levers; the first magnifies a hundred times, and the second enlarges the first a hundred-fold, the total magnification being thus ten thousand times. The difficulty introduced by the weight of levers was surmounted by the employment of navaldum, an alloy of aluminium, which combines great rigidity with exceptional lightness. The friction at the points of support was removed by the employment of jewel bearings. The record is taken on a smoked glass plate kept oscillating to and from by means of a crank K and eccentric R, actuated by a