Cee ar 
eee eee a eee 
1897 | THE MOVEMENTS OF DIATOMS 43 
cially when it is spring water. Gradually, during the time of 
the observation, the movements became less active, and at the 
end—if the drop is not changed —they cease altogether. That 
is quite explicable by the contained gases in fresh and cold 
water, which are gradually exhausted or evaporated, in conse- 
quence of the warming of the drop. This observation gives 
equal support to both of the functions proposed as causal. 
But there are some cther observations which seem decisive 
in favor of the assimilation being the real cause of movements. 
These are: 
1. The dependence of movements upon light.—It is impbdssible 
to make direct observation in darkness, as in these conditions 
we could see nothing ; but the following experiment speaks for 
the above stated dependence: If, while observing a cell in move- 
ment, we suddenly cover the mirror so as to intercept all the 
rays of light, upon uncovering of the mirror after some minutes 
we generally find the cell at rest not far from the place where it 
was seen before the light was cut off. It does not start forward 
at the moment when light is again admitted, but for some time 
seems to be in a state of rigor and then gradually begins to 
move. That little experiment was repeated many times, and 
always with success. 
2. Lhe dependence of the movement upon the color of the light.— 
I used for this purpose the light transmitted through solutions 
of ammoniated copper oxide and of bichromate of potassium, 
both 1™ thick. As shown by spectroscopic analysis the first 
allowed the more refrangible rays from the middle of green to 
pass ; the second transmitted the opposite part of the spectrum. 
The blue raysdo not promote the motion; the red ones pro- 
mote them very energetically. By means of such rays I suc- 
ceeded in setting in movement some big cells of Pinnularia, 
Surirella, and others, quite immobile in the daylight. 
3. Lhe dependence of movements upon the direction of light.—li 
we remove slowly the slide carrying a diatom moving in the 
manner described, so as not to allow the cell to cross a plane 
passing through the apex of the light cone perpendicular to the 
