1 18 'Journal of Comparative Neurology and Psychology. 



near the side farthest from the light. In some conditions, the 

 colonies thus introduced, proceed across the aquarium nearly 

 parallel with each other, spreading but little, frequently not more 

 than five or six millimeters. In other conditions, however, they 

 spread out as much as three or four centimeters. By laying a 

 straight w^ire on the aquarium and constantly keeping it over the 

 middle of the group of colonies as they proceeded on their way, 

 the average course was quite accurately ascertained. The course 

 under most conditions, although at a decided angle with the rays, 

 was remarkably straight; but under some conditions it curved con- 

 siderably as the organisms approached the side of the aquarium 

 nearest the light. The paths produced and the direction of the 

 rays were transferred to paper by means of a miter square; and 

 thus the angle of deflection was recorded for future reference. 



Between August 20 and 30, 1904, seventy-three paths observed 

 under the conditions described above were recorded, and many 

 more were observed. In nearly all of these cases the colonies 

 deflected strongly to the left, frequently making an angle of 45 

 degrees with the light rays, and rarely less than 5 degrees. This 

 deflection to the left was brought out in a striking way, by putting 

 the Volvox colonies into the aquarium a few centimeters to the 

 right of the left edge of the light area. When introduced at this 

 point, they soon reached the plane between light and shadow and 

 passed into the dark area without any apparent change in their 

 course. After they had traveled in the dark region some little dis- 

 tance they rose, deflected more sharply to the left, and frequently 

 made a small circle or spiral and entered the light region again. 



This deflection to the left in the light area was, of course, thought 

 to be due to the fact that the light was graded in intensity, the 

 more intense end of the field being to the right. It was accident- 

 ally discovered, however, that similar deflections were produced 

 when no lens was used, and later it was found that when the screen 

 over the lens was inverted so as to make the left end of the field 

 the more intense instead of the right as in the previous experiment, 

 the Volvox colonies still deflected to the left. It was therefore 

 clear that this deflection was not due to difference in light intensity 

 on opposite sides of the organisms. 



One hundred and one additional paths were observed and 

 recorded between July 18 and August 3, 1905. Some of these 

 observations were made in the light grader; others outside. In 



