HIPPOLYTE VARIANS. 599 



convenient height with water ; the " water-circulator " dishes 

 are filled completely with water, the glass plates are carefully- 

 slid on, and thus each o£ these dishes comes to be, from the 

 physical point of view, a mere dilatation of a long tube 

 completely filled with water. 



Suction is now applied at the open end of the exit-tube 

 from the last '' water-circulator," air passes through the air- 

 circulators into the aspirator pari passu with the outflow 

 of water into the water-circulators. The rate of flow is 

 determined by the difference in level between the lower end 

 of the pressure-tube and the free end of the exit-tube from 

 the last water-circulator (x^ y). By raising or lowering this 

 latter tube the rate is varied. The rate which we found 

 most favourable for the cultivation of our Hippolyte was 

 from 12 c.c. at night to about 65 c.c. per minute during the 

 day, a rate which necessitates the refilling of the aspirator 

 some seven times a day. By means of an arrangement of 

 tubes fitted with two-way cocks, any single '' water-" or ''air- 

 circulator " can be cut off without interfering with the circu- 

 lation through the others. 



The form of the " water-circulator " dishes described above 

 was chosen in order that experiments on the influence of 

 monochromatic light upon the colour-changes in Hippolyte 

 might be made. To this end we employed the original, as 

 well as a modified form of Landolt's " Strahlenfilter," which 

 consists of a cylindrical vessel with glass ends and metal 

 side, glass plates accurately dividing the chamber into two 

 or three compartments of required depths. These compart- 

 ments are filled, through openings in the metal side of the 

 Strahlenfilter, with the coloured solution made to Landolt's 

 prescription (see 1894). On some occasions, to ensure high 

 intensity, light from the incandescent gas burner was re- 

 flected through the liquid colour-filter into the " water-circu- 

 lator" beneath. 



The colour-filters were, in our hands at least, eminently 

 unsatisfactory, and a more convenient form of light-filter is 

 much to be desired (see Nagel, 1898). 



