508 S. O. MAST 



quently the more nearly equal the stimulating effect of the light 

 in the two beams is when the organisms follow a line bisecting 

 the angle between the beams. The degree of positiveness or 

 negativeness in these creatures varies with the intensity of the 

 illumination and with the time they are exposed. In the be- 

 ginning of a series of tests they usually orient more precisely 

 and deflect less than they do later. We should consequently, 

 under any given condition, expect the sector readings to increase, 

 and this is precisely what was observed as stated above. 



This difficulty was in part eliminated by taking for each in- 

 dividual test fresh specimens from a culture kept under con- 

 stant f^onditions, and the most accurate readings were obtained 

 in this way. The results of one such series of tests are given in 

 columns 2-5, table 10, but such a series is by no means always 

 obtainable, for very often the response of the colonies is too in- 

 definite. This is clearly illustrated by the results of many pre- 

 luninary tests made preceding those just mentioned as shown 

 by the following abstract from my note book. 



The specimens used in this series of tests selected to illustrate 

 the point in question were collected late in the evening of Au- 

 gust 11. They were very abundant and apparently in excellent 

 condition. Two half -liter jars full of solution containing them 

 were taken to the laboratory and placed in a north window. The 

 following day colonies taken from these jars were repeatedly 

 examined and tested under various conditions. Some were put 

 into tap water, others into pond water; some were left in dark- 

 ness, others in strong light, for various periods of time. More- 

 over, they were tested in illumination varying in intensity from 

 very high to very low and in temperature varying from the 

 maximum well toward the minimum, but in all cases orientation 

 was too indefinite and movement too slow to obtain even moder- 

 ately accurate results. Practically the whole time from early in 

 the morning until three o'clock in the afternoon was spent in 

 these fruitless attempts. During this time the two half-liter 

 jars containing the colonies were continuously in the north 

 window, but they were subjected to greatly varying conditions 

 of illumination for this window was in the room in which the 



