Mast, Light Reactions in Loiver Organisms. 171 



tact with the wall of the jar nearest the light, more definitely than 

 do the smaller ones. The cause of this has been discussed else- 

 where (pp. 128-13 1). 



It was found by Oltmanns ('92, p. 191) that the optimum light 

 intensity for Volvox changes during the day. He discovered on 

 August 4, that the colonies aggregated in a darker part of the 

 aquarium at 4.30 a. m. than at 8.30 a. m., although it was not 

 yet daylight at 4.30. On another day, however, the aggregation 

 was found in a still darker region between 11 a. m. and 5.30 p. m., 

 and this day it was found in a region slightly lower in light inten- 

 sity at 5.30 p. m. than at 12 m. in spite of the fact that the sunlight 

 was unquestionably stronger at 12 than at 5. Oltmanns thought 

 this variation in optimum intensity to be due to a periodicity 

 analogous to that found in higher plants. I found no evidence of 

 such periodicity. The change in position during the day noted 

 by Oltmanns corresponds to change in the sense of reaction, which 

 can be induced at any time of the day by exposure to light of proper 

 intensity. I did not, however, go into detail with reference to 

 this point; it is therefore desirable to have more experimental 

 results along this line before coming to definite conclusions. 



18. weber's law. 



"On comparing objects and observing the distinction between 

 them, we perceive, not the difference between the objects, but the 

 ratio of the diflFerence to the magnitude of the objects compared" 



(TiTCHENER, '05, p. xvi). 



This law was formulated by Weber in 1834 with especial refer- 

 ence to the senses of touch and sight. Davenport ('97, p. 43) 

 has worded it as follows: "The smallest change in the magni- 

 tude of a stimulus which will call forth a response always bears 

 the same proportion to the whole stimulus." 



By means of his well known capillary tube method Pfeffer 

 ('84) proved the law to hold approximately for the reactions of 

 fern spermatozoids to malic acid, and later ('88, p. 634) also 

 the reaction of Bacterium termo to meat extract. Massart ('88) 

 proved it to hold for the light reactions of Phycomyces, by placing 

 the plants between two flames and thus obtaining the minimum 

 difference in light intensity on opposite sides which induced a 

 response. He found the minimum intensity difference to be 18 

 per cent of the total light intensity, and this held true for all degrees 



