MOVEMENTS IN THE VISUAL CELLS 177 



pigment, a series of experiments, chiefly upon Ameiurus, were 

 performed. 



In experiments involving a reduced oxygen supph-, the ap- 

 paratus was simple. A 3| litre flask was filled with boiling 

 water. The flask was then closed with a three-hole rubber 

 stopper through which passed, (1) a glass tube extending to 

 the bottom of the flask, which served to introduce gas from a 

 hydrogen generator, (2) a glass overflow tube extending about 

 three-quarters of the way do^v^l the flask, which served chiefly 

 as an outlet for the hydrogen gas, (3) a mercurj' pressure regu- 

 lator. As soon as the flask of boiling water was stoppered, the 

 hydrogen supply was turned on and as a result, water was 

 forced to escape through the overflow tube, its place being taken 

 by hydrogen gas. When the water level reached the bottom 

 of the overflow tube no more escaped, l:)ut the gas after bubbling 

 through the water did do so and was conductetl through a water 

 trap to the outside air. As the water cooled do\\Ti to room 

 temperature it could not take up oxj'gen since none was present , 

 and furthermore, the bubbhng hydrogen gas tended to expel 

 mechanically any residual oxygen present in the boiled water. 



Water containing an excess of oxygen was j^repared by bub- 

 bling oxygen gas through water in a flask similar to that de- 

 scribed in the former ex])eriment, whence it escaped by means 

 of an overflow tube loading into a water trap. The wat^r used 

 had previously been boiled and reoxygenated by an aquarium 

 aerating device. 



Quantitative determinations of the oxygen content were made 

 at the expiration of all experiments by the method of Winkler 

 (Trcadwcll and Hall, '05). 



Ameiurus was used in most of the experimentation, although 

 Abramis served in a few cases. The description which follows 

 applies particularly to Ameiurus. 



It was possible to reduce the oxygen supply to an amount 

 in which the fish could not live.^' This, for example, happened 



'5 The normal oxygen saturation of water at 20°C. is 6.356 cc. per liter (Tread- 

 well and Hall, '05). Boiled water which had been cooled rapidly was found 

 to contain about 0.93 cc. per liter. 



