188 JACQUES LOEB AND HARDOLPH WASTENEYS 



observations was limited and it seemed desirable to continue 

 these experiments, since it is of fundamental importance to know 

 whether or not apparently purposeful instinctive reactions such 

 as the tropisms can be expressed in terms of purely physico- 

 chemical laws. 



The method followed in the work of Loeb and Ewald consisted 

 in ascertaining the time required to cause 50 per cent of the 

 polyps of Eudendrium to bend towards the light, and it was 

 found that this time varied inversely with the square of the dis- 

 tance of the light from the animals. We found that this method 

 could not be followed with satisfaction on account of the great 

 variation in the quality of the material from day to day. 



We therefore selected another method. We confined our ex- 

 periments to three intensities of light by putting the specimens 

 at distances of 25, 37.5, and 50 cm. from a Mazda incandescent 

 lamp of about 33 Hefner candles. The times of exposures were 

 adjusted so that on the assumption of the applicability of the 

 Bunsen-Roscoe law the same effect, i.e., the same percentage of 

 polyps bending towards the light should be produced. Thus in 

 some experiments the exposure for the three distances given 

 was 10, 22.5 and 40 minutes respectively, in others, 7, 15.75, and 

 28 minutes, and so on. The ratios of the percentage of polyps 

 bending towards the light for the three distances should be as 

 1 : 1 : 1. Since the material differed widely in different experi- 

 ments and in different dishes, it was necessary to compute the 

 averages of a large number of experiments. 



The source of light was, as stated, a Mazda incandescent 

 electric light of about 33 Hefner candles. This was screened 

 with a series of black screens having circular openings of about 

 7. 5 cm. in diameter to prevent reflection of stray light. 



The colonies, immersed in sea water, were arranged in a row 

 in rectangular glass dishes, the stems being inserted in holes 

 made in a layer of paraffin mixed with lamp black as in the 

 previous experiments. The rear side of the dish was also coated 

 with the paraffin lamp black mixture in order to prevent reflec- 

 tion of light from the slightly uneven back surface of the dish. 



