258 COHN, ON THE MOVEMENTS 
underneath. But if the window side of the drop be entirely 
shaded by an opaque body, then the organisms pass away 
from the window edge and go to the room side. If the light 
from above be entirely impeded, and the drop be only lighted 
from underneath by the reflector, the organisms will assume 
no particular position, but exbibit disorderly movements 
equally throughout the drop. The same takes place if the drop 
has been some time completely in the dark; but if, on the 
contrary, when the light which comes from ates 1S shut off, 
only a part of the drop be lighted from underneath by means 
of the reflector (through the ‘apposition of a diaphragm which 
is smaller than the drop) all the organisms will swim towards 
the lighted point. If, for example, this point is in the 
middle of the drop, they will leave the edges and will crowd 
together in the middle of the drop. 
If a basin be filled with water which contains numerous 
green organisms, they will congregate at the window side; 
but if this be shaded by an opaque plate being placed upon 
it, they will go away from the window side and collect 
towards the opposite side; and, indeed, they will often place 
themselves in a dark green line, obliquely through the 
surface of the water, on the boundary of the shadow which 
the plate has thrown. 
From the experiments that have hitherto been mentioned 
one might come to the conclusion that it is the intensity of 
the light that rules the movements of the green organisms, 
and that they prefer the window side to the room side because 
it is strongly lighted. If so, it must be at once granted that 
a sensibility exists in these ‘animals which can perceive the 
inexpressibly small difference of brightness between the two 
edges in a drop of only one millimetre. But in this way we 
could not explain the reason of the powerlessness of the light 
from underneath transmitted by the reflector, compared 
with the light falling from above, and still less the reason for 
their preferring the window edge, when the light is palpably 
weakened by a semitransparent body, to the light which is 
given by the reflector when working with all its intensity, 
and is therefore the stronger. 
Further researches have proved that it is not the intensity, 
but the direction of rays of light, which governs the move- 
ment of microscopical organisms. All the above experiments 
were made in a room where the light fell on one side, and 
the drop was flat, and which only caused one direction of 
movement. In such a case the or ganisms always move them- 
selves towards that edge which is turned to the source of the 
light. But in open air, where the light falls on every side, 
