18 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
two-thirds to three-fourths of the way through the second cylinder, and 
from that onward to the concave surface of the second cylinder the light 
was diverging. 
By this arrangement any organism moving along the path of the rays, 
either towards or away from the light, is forced in one part of its path to 
travel in converging light, and the remaining part it travels in diverging 
hght. Experiments were carried out both with white he@ht and with 
coloured lights. The first set of organisms examined were negative; these 
swam away from the heht into light of cncreasing intensity towards the 
caustic, and then through this onward in heht of decreasing intensity till 
they reached the glass surface most remote from the light. Positive 
organisms were next tried, and swam in the exactly reverse direction, first 
from the most distal part towards the caustic in converging light, and 
therefore of increasing intensity, and then onward in diverging light, 
therefore of decreasing intensity, up to the glass surface nearest to the 
heht. 
At first sight 1t looks proven from this that intensity of h@ht is of no 
effect, and the direction of incidence the whole matter, because the 
organisms appear to swim in one direction indifferently, whether the 
illumination is increasing or decreasing. In reality, however, such a 
conclusion would be fallacious, for in order that, say, a positive organism 
should turn when it began to swim in light of gradually decreasing 
intensity, it would be necessary for it to turn its sentient surface away 
from the hight, and that would plunge it into darkness. 
The true conclusion is shown by what might be termed secondary 
effects seen on carefully watching the above experiment with negative 
organisms. These organisms at first accumulate in the narrow band of 
light at the distal glass surface from the hight, where they dart about in 
small curves, keeping close to the glass; but in a few minutes it is found 
that a great many of them have accumulated in the two shady margins 
just outside this strongly illuminated band, and on either side of it. The 
probable explanation of this is that for these negative organisms the 
feebler light outside the band is nearer the optimal stimulus, and when 
they escape from the direct light beam in the course of their 
