the mechanical action of light, under favourable conditions; yet what imagination can picture to itself the force of a single vibration of such light, millions of millions of which strike in a second upon the retina, without causing even a sensation, or only the feeblest. But if Tyndall be right, such force repeated, inconceivably small as it is, is sufficient to clothe our forests from spring to spring, and to fill our fields and vineyards with ample store of food. If we cannot as yet dogmatically assert "ubi vero lumen, ibi quoque vita," we do thus by every advance of knowledge more clearly perceive that the physical existence, as well as the intellectual development of living things, was involved in the first fiat, "Let there be light."
To turn from these considerations to such as more immediately concern us, let me speak now of the progress which has been made in our knowledge of the nervous system.
Since the discovery of the circulation, no greater work has probably been done in phy-
