VARIATIONS IN REACTION To LiGH. 5 
more or less towards the same direction as it darts about under other 
varying influences and stimuli, and this on the whole gives steering to the 
course, so that the animal as a net result moves towards or away from 
the light. 
Taking this movement then as a sign of chemical change in the cells 
of the organism, or certain of those cells, the effects were observed—of 
exposure of organisms to light of varying intensities, of change in 
reaction as a result of keeping in light of about constant intensity, of 
velocity of movement in light of varying intensity, of the effect of light 
of different colours, and of velocity. of movement in such lights, of the 
effects of converging and diverging light, of the effects of ight and shade 
on organisms in the same vessel, on the association of upward or down- 
ward movements in level with positive or negative phototaxis, and on 
movement in presence of more than one source of light. 
A very considerable literature exists dealing with heliotropism and 
phototaxis, but no attempt need be made to quote from this, further than 
relates to the organisms used for the research, or in incidental relationship 
to the variations in reaction to light described in the present experiments.! 
The experiments were made with a free-swimming larval stage of the 
Barnacle (Nauplii of Balanus), obtained in by far the largest quantity in 
the tow-nettings, mixed with a much smaller number of copepods, and 
larval spirochaetes. 
The manner in which the organisms congregate at the points of the 
dish nearest to and farthest from the light was used to pipette them off 
and separate them from other organisms indifferent to the hght, and the 
positive and negative groups of organisms so obtained were examined 
separately. Many of the Naupli were found in both the positive and 
negative groups, but no difference in average size or degree of development 
could be found in the two types to differentiate them, and later experiment 
showed that the same separated group might be artificially varied back- 
ward and forward between positive and negative according to their 
previous treatment by light. 
1. For a general survey and for literature, reference may be made to Verworn, General 
Physiology, translation by Lee, 1899; Holt & Lee, American Journal of Physiology, Vol. IV, 
p. 460, 1901 ; and Loeb, Dynamics of Living Matter, 1906. 
