Reactions of I so pods to Light 283 



'06, p. 387, footnote) a layer of distilled water 9.21 mm. thick 

 transmits only 11 per cent of the total incident radiation. Tap 

 water transmits only a very little more. Hence the amount of 

 heat reaching the bottom of the above mentioned tank would be 

 less than 0.2 per cent of the total incident radiation." 



To guard against a rise in temperature of the water in the 

 experimental (inner) tank, the water of the outer tank was con- 

 stantly renewed by siphons, one drawing off the water at one end 

 while the other replenished the tank at the other end. This 

 arrangement proved effective. 



The intensity of illumination at the bottom of the tank, allow- 

 ing \ per cent reduction in actual luminosity for each centi- 

 meter of water through which the light passed (see page 277), 

 was 6983 CM. (772 c.p., 6-glower Nernst lamp at 30 cm. from 

 surface of water, which was 3 cm. deep). The luminosity in the 

 dark region of the tank, while not determined, was apparently 

 near, though below, the threshold of stimulation for directive 

 response in Asellus, and certainly much below that in Caecidotea. 



Sometimes the experiment was started by placing the animals 

 in the illuminated part of the tank and observmg their reactions 

 at once. At other times the animals were placed in the tank and 

 allowed sufficient time to become thoroughly settled before they 

 were exposed to the light. Separate treatment of the experi- 

 ments on the basis of this difference is unnecessary, for the dis- 

 advantage of the random movements of the animals in the former 

 case was balanced by the disadvantage of the apathy of the animals 

 in the latter case. In both the ultimate responses were the same. 



Numbers of individuals were experimented with at the same 

 time, as in the experiments with horizontal illumination. 



- If the first 9.21 mm. of water transmits 11*^1 of the incident radiation, the second 9.21 mm. would 

 transmit 11 per cent of its incident radiation, i.e., 11 per cent of 11 per cent or 1.21 per cent of 

 the incident radiation at the surface of the water. 



The third 9.21 mm. of the water would transmit 11 per cent of its incident radiation, or 0.133 P^"" 

 cent of the incident radiation at the surface of water. 



The remaining 2.37 mm. of water would still further reduce the amount of heat reaching the bottom 

 of the tank. 



