16 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
Experiment IV .—Selection of position under different coloured glasses 
with the same direction of incidence. 
In this experiment, diffuse daylight was used on some occasions and 
candlelight on others, the long side of the dish being placed parallel to the 
surface of the window, or next to the candles. Then the two glasses of the 
two different colours to be compared were placed edge to edge, each covering 
one-half of the dish, the edge where the two slips of glass touched being at 
right angles to plane of window, so that each half was situated exactly the 
same as to direction of incidence and intensity of light from the window ; 
and a similar arrangement was used with the candlelight, the candles 
being so placed opposite the middle of one of the long sides of the dish that 
they shed equal light on the two different coloured halves. Before placing 
the two slips over the dish, the contents were stirred so as to uniformly 
distribute the organisms, but care was taken that the contents were not 
rotating when the slips were put over. Also, after the organisms had 
distributed themselves selectively, and the result had been noted, the two 
slips were reversed in position, each to each, and the change in distribution 
observed; the organisms at the time were strongly positive in the candle- 
light, and strongly negative in the diffuse daylight. 
First, using four candles in the dark room, and with the red glass on 
the left-hand half and the blue glass on the right-hand half, in 2 min. 
30 secs. from the commencement all the organisms are under the blue 
glass and next the candles, none under the red glass. The red and blue 
glasses are now reversed without disturbing candles or organisms, and in a 
very short time all the organisms have shifted and are once more under 
the blue glass in its new situation. 
Second, similar results obtained with diffuse daylight, except that 
organisms now swim from the light; with blue and red most of the 
organisms under blue, a few only under red; with blue and green, two 
groups form at the two corners distal to the light, the larger of the two 
groups being under the green. Thus the organisms move with equal 
velocity under all coloured glasses, but when two colours are offered for 
selection they accumulate chiefly under one. Further, the direction of 
movement to pass from one colour to another is across the direction of 
