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In discussing the difficulties connected with investiga- tions relating to longitudinal growth and its variations, Special stress must be laid on the importance of maintaining external conditions absolutely constant. This constancy can only be maintained in practice for a short time. Lengthy periods of observation, moreover, introduce the uncertainty of complication arising from spontaneous variation of growth. The possibility of accurate investigation, therefore lies in reducing the period of the experiment to a few minutes during which we have to determine the normal rate of growth and its variation under a given changed condition. This^ would necessitate the devising of a method of very high magnification for record of the rate of growth.*

With auxanometers now in use, which give a magnifica- tion of about twenty times, it takes nearly four hours to determine the influence of changed condition in inducing

" A short account of my researches with the High Magnification Crescograph has been published in the Proceedings of the Royal Society. I shall in the following Papers give a detailed account of my investigations on growth and on allied phenomena.