8 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
become negative, and keeping in darkness or in a feeble illuminatiorx 
causes this negativity to pass back to a positive phototaxis. 
This again is compatible with the view that the effect of light upon 
the sensitive substances of the organism is always the same whether the 
effect is shown by a positive or negative phototaxis. ‘The degree of the 
stimulus determines the reaction of the organism towards it, as shown by 
the direction of the orientation and consequent movement, but the 
chemical nature of the stimulus is the same. Below a certain optimum 
the organism reacts so that the sensitive surface is turned towards the 
light, that is to say so as to increase the amount of light energy reaching 
it, and so increase the reaction towards its optimum value for the organism 
in its condition at the given moment. Above the optimum value of 
stimulation, the organism conversely reacts so as to turn the sensitive 
surface into a region of diminished light intensity, and so also to decrease 
the velocity of the reaction towards its optimum for the organism. 
This supports the view expressed by Holt and Lee, that direction of 
light is only effective in a secondary manner in so far as it alters intensity 
of hight falling upon different parts of the organism, and the orientation 
is hence primarily a question of intensity of hight. 
The very ingenious experiment of Loeb, showing that an organism 
which is positively phototactic to direct sunlight will pass from this 
onward into diffuse sunlight, that is, into a region of lower intensity of 
illumination, and will not reverse its direction when it finds itself in this 
region of lower illumination, is quite susceptible of explanation on this 
view, as well as the result of the experiments given below in this text, 
upon the movement of negatively phototactic organisms away from the 
source of illumination in converging light, and still onward past the focus 
of the light in now diverging hght with decreasing intensity. 
Loeb’s experiment consisted in placing an organism (young cater- 
pillars of Porthesia chrysorrhaca) im a test-tube the axis of which was 
horizontal and at right angles to the plane of a window near by, through 
the upper part of which direct sunlight fell on the more distal portion of 
the test-tube, while the portion of tube near the window was lit only by 
1. Loc. cit. 
