VARIATIONS IN REACTION TO LIGHT. 1 
incidence, and not to or from the light, and the relation to the light of the 
two halves being the same, it would appear that a preference for a 
particular colour or wavelength (or the greater or lesser stimulus of 
‘different wavelengths), caused the different distribution. If the organisms 
are carefully watched when they are becoming distributed, it is seen that 
they do not move directly across from one half to the other, but are moving 
about apparently freely, an organism every now and then leaving a group 
and darting off; but there is a certain amount of steering and controlling 
during these apparently free movements, which ultimately settles them 
down in their final distribution. 
In this type of experiment, observation of the grouped animals shows, 
as in all the other experiments where the animals are grouped either 
positively or negatively under the influence of the light, that there is no 
such thing as fixed and continuous orientation of the minute animals. In 
every group a great many are moving about in and out amongst one 
another, and a good many are entering and leaving the group like bees 
from a hive, but each individual, after a short trip about soon returns to 
the group. ‘The source of light is, in fact, a strong directive influence, but 
there is no rigidly fixed orientation, any more than there is in a cluster of 
midges, or a brood of chickens around their mother. 
Experiment V.—Movement in converging and in dwerging light. 
In order to obtain converging and diverging light, cylindrical 
museum jars, about 10:5 centimetres in diameter and 18 centimetres high, 
were used, which happened to be in stock at the Station. 
Two such jars were used; the first, filled with clear fresh water, was 
used only as a cylindrical water lens, and contained none of the 
organisms; the second jar contained the organisms in sea-water. The 
first cylinder was placed a variable short distance, up to about one foot, 
from a small oil lamp with a circular wick, and the second cylinder was 
placed close up against it, on the other side from the lamp. The lamp and 
two cylinders were so arranged that the diverging light from the lamp 
became slightly convergent in passing through the first cylinder, and being 
still further converged by the second cylinder, it formed a caustic about 
B 
