VARIATIONS IN REACTION TO LIGHT. 3 
before there is much increase in the temperature of the sea—an outburst 
upon which the whole life of the sea is as thoroughly dependent as that 
of the terrestrial world is upon the similar outburst of activity in land 
plants; and in the most marked movements which occur towards or from 
the light according to varying circumstances of the minute organisms, 
either larval or adult, which chiefly constitute the plankton or floating 
life of the ocean. 
It is hence clear that the observation of the reactions of living cells 
to light is of importance both to the student of biology, and to the student 
of medicine who makes practical applications of the discoveries of biology, 
using the term in its widest sense. 
Recent discoveries have proven the value of light treatment as a 
practical adjunct of medicine, and the study of light effects upon the 
simpler organisms must sooner or later yield a key, both for the rational 
understanding of such effects, and their extension to further utility. In 
addition to these utilitarian advantages, the study is one of the most 
fascinating from its own intrinsic interest in the whole wide field: of 
biology. 
One of the most obvious lines of attack in investigating the reactions 
of living organisms to light is the study of the movements of the 
organisms, either as a whole in the case of freely moving organisms, or 
the change in relative position, or orientation, in fixed or sessile 
organisms. 
Tt must, however, be clearly borne in mind that this movement is an 
index of other things, that the underlying problem is ultimately and 
essentially a chemical one, or, better expressed, one of chemical 
transformation of light energy.!_ The organisms move because of an action 
of light upon chemical constituents in the cells, that is to say, there is a 
change in the metabolism of the cell stimulated, giving rise to the 
movement of the organism. Also, according to the nature and cendition 
of both cell and light-stimulus, which form the two inter-ectirg factors, 
the character and sense of the movement of the organism will vary. 
1. This view has been also put forward by Lceb, Dynanics cf Liv.ng Maiter, 1906, pp. 112 
et Seq. 
