Mast, Light Reactions in Loiver Orgajjisms. 1 17 



An aquarium' 15 cm. long, 8 cm. wide and 8 cm. deep inside 

 was placed at one of the principal foci in the light grader, which 

 was in a horizontal position and so arranged that the Nernst 

 glower at the other principal focal point was vertical. The light 

 rays passed through the aquarium practically parallel with each 

 other and the bottom of the aquarium and perpendicular to the 

 sides. An opaque light screen, containing a rectangular slit 10 

 mm. wide and 13 cm. long, wasfastened to the sideof the aquarium 

 nearest the light, so that the lower edge of the slit was on a level 

 with the bottom and the ends of the slit were i cm. from either 

 end of the aquarium. Filtered water was poured into the aqua- 

 rium to such a depth that its surface was above the upper edge 

 of the slit and was consequently in darkness. Since the rays 

 were practically parallel with the bottom and perpendicular to 

 the sides of the aquarium, it is evident that reflection from the 

 watersurfaces was practically eliminated. The light which passed 

 through the aquarium was largely absorbed by the wall of the 

 dark room which was over seven meters from the light grader, 

 and since this was the only light which entered the room it is safe 

 to conclude that the direction of movement of Volvox" in the aqua- 

 rium was influenced only by rays direct from the Nernst glower. 



By placing an opaque screen containing a triangular opening 

 over the cylindrical lens in the light grader, a field of light is 

 produced which becomes gradually less mtense from one end to 

 the other (Mast '06, p. 364). If Volvox is allowed to swim 

 toward the source of light in such a field it is evident that one side 

 of the colonies will be more strongly illuminated than the opposite, 

 and if difference in light intensity on the two sides, regardless of 

 ray-direction, determines the direction of movement we should 

 expect the organisms to move at an angle with the direction of the 

 rays of light. This was found to be true, as will be shown later 

 (pp. 136-141). 



The first series of experiments made to ascertain the effect of 

 difference in intensity on orientation was performed in the light 

 grader, arranged as described above, by carefully introducing 

 about one hundred colonies into the aquarium at a fixed point 



' The aquarium was made of the best plate glass obtainable, accurately cut and ground, and cemented 

 with Canada balsam boiled in sufficient linseed oil to give it the desired consistency. This cement 

 proved very satisfactory, much more so than any other of several tried. Balsam in xylol is good but 

 it becomes so brittle on drying that it breaks readily. Linseed oil prevents this. 



