COHN, ON MICROSCOPICAL PLANTS. 297 
the glass vessel. In the same manner diatoms are found in 
every part of an aquarium, on the surface of the soil as well 
as on the walls, but are never met with in its deeper and 
darker parts. 
Numerous experiments with green microscopical organisms, 
especially with Euglena, gave the following results :—If a 
drop of water, which is thickly and equally filled with micro- 
scopic organizations, be placed upon a glass slide, it will be 
seen, before many minutes, that many of the organisms will 
betake themselves to that portion of the drop which is turned 
towards the window, or even towards that part of the sky 
which is most lightened. ‘They crowd around this side, 
which we may call the window side, and give the drop a 
deep green edge, whilst the rest of the drop is quite colour- 
less and free from Euglene ; and, indeed, they place them- 
selves together, so that their heads le parallel one to another 
towards the light, and their bodies are directed perpendicu- 
larly to the edge of the window. They cannot remove them- 
selves from this position, but gradually dry up as the edge 
becomes evaporated. 
If, conversely, the drop be now turned, so that that which 
was formerly the window side is now turned away from the 
window and directed towards the room, and so that the 
former room-side forms the window-side, an instantaneous 
struggle in the whole of the organisms will be seen for the 
purpose of turning themselves round. The foremost soon 
turn round and swim towards the new window edge, and the 
back ones follow one after the other. After two or three 
minutes, more or less, as they are tree to move, the organisms 
are again at the window edge. This experiment can be 
repeated as often as one likes; the result is the same whether 
the drop lies on a dark ground or is lighted from beneath 
through the diaphragm. 
If the drop be placed on the microscope stand, so that the 
half which is turned towards the window lies on a dark 
ground, and, on the contrary, that which is turned away 
from the window be lighted from underneath by the reflector, 
then it will be seen that the organisms swim towards the 
window edge, although the other half of the drop seems to 
receive more light, seeing that it is lighted from above and 
underneath at the same time. And even when the light 
which comes from above is weakened by placing a semi- 
transparent body between it and the object—such as a thin 
piece of horn or oil paper—the objects will always go towards 
the window edge, and do this even when the full light of 
the reflector is thrown at the same time upon them from 
