Of all the processes proposed to measure varying luminous intensities by means of chemical effects, not one has yet been expressed in strictly dynamical measure. This is owing to the very small amount of energy to be measured necessitating very peculiar processes for its recognition. The chemical actions generally induced by light are of the “Trigger” or “Relay” description; that is, bear no necessary relation to the power evolved by the transformation. There is one natural action of light continuously at work of a very different kind in the decomposition of carbonic acid by plants, necessitating a large absorption of energy, and thus enabling us to ascertain the proportion of the radiant power retained, through the chemical syntheses effected.