But a few years ago the diverse phenomena of optical activity, such as the changes of rotation which occur with alteration of temperature, of colour of light, of solvent, or of concentration in a solvent, appeared, in spite of the great accumulation of relative data, to be practically independent of each other, and gave very little hope of satisfactory generalisation. Quite recently, however, the possibility of bringing into one scheme all these different branches of the subject has become more than a mere aspiration, and the progress which has been made by several investigators reveals clearly the existence of a deep-seated and farreaching regularity underlying the remarkable sensitiveness to external conditions of the phenomena in question.