VARIATIONS IN REACTION TO LIGHT. 15 
This experiment on the velocity of movement in light of different 
intensity and of different colour, was made by observing the time required 
for the organisms to swim from one end to the other of a flat, black 
vulcanite dish of rectangular shape. The length of the dish was 17 cm., 
and the organisms were first brought to a compact mass at one side by 
placing the light to be used at one end and then the time noted for them 
to swim across and form a similar compact mass at the other end when the 
source of light was shifted to that end. 
Then the time was again noted which they require to swim back to 
: their original position; these times are denoted by ‘Out’ and ‘ Back’ 
‘the following table. In taking the time ‘ out’ one does not wait for every 
organism, but waits till the great majority are in a compact group, this, 
after a little practice can be done accurately within a quarter of a minute 
or less. 
For different intensities of light, one ordinary paraffin wax candle 
Was used in one case, and four similar candles in the other case. For 
white light, the dish was simply uncovered, and for coloured lights it was 
covered completely over with slips of coloured glass, through which the 
coloured light passed to reach the organisms. The coloured glasses 
which it was possible to obtain were red, green and blue. Regarding the 
total intensity of light passing through the three slips, it appeared to the 
eye as 1f the red strip was most obscure, and the green most transparent, 
the blue being intermediate, but no exact photometric instrument was 
available. The organisms used were a strongly positive group obtained by 
pipetting off in diffuse daylight. 
1. Illumination intensity = one candle. 
Pedegehn 2... Lime Out’ ... 3 min. 0 secs. 
Time ‘ Back’ ... 3 min. 0 secs. 
ivetieht)... Lime “OQut’ ... 3 min. 30 secs. 
Time ‘ Back’ ... 3 min. 20 secs. 
Green light... Time ‘Out’ ... 3 min. 40 secs. 
Time ‘ Back’ ... 3 min. 0 secs. 
2. Illumination intensity = 4 candles. 
White light... Time ‘Out’ ... 3 min. 0 secs. 
Siimer Back; 7.) 2 min. o0 secs. 
ifeaten; =.. lime Out’ ... 3 min. 0 secs. 
Time * Back’ ... 3 min. 0 secs, 
