A. R. MOORE AND W. H. COLE 



333 



experiments this cage containing twelve female beetles was kept in a 

 thermostat made of glass at a temperature of approximately 38.5°C. 

 with extreme variations for short intervals on two or three occasions 

 during the season to 37.5 and39.5°C. Five intensities of illumination 

 were secured by the use of Mazda glowers of different powers. Dur- 

 ing the experiment the glower was held by a clamp 25 cm. from the 

 top of the cage. The latter was protected from the heat of the lamp 

 by a water screen. In making the observations one observer manipu- 

 lated the cage by inverting it for each start and then noted the instant 

 that six beetles (50 per cent of all) reached the top of the cage. The 

 start and stop were signalled to the second observer who kept time 

 with the stop-watch and recorded the time intervals to the nearest 



TABLE II.* 



* I = the intensity of the light in candle meters, R = rate at which the organ 

 isms respond, k = 1.09, and K = 2.71. 



second. Five observations were made at each intensity with each 

 lot of twelve animals. Seven such series were run. Each point is 

 therefore the result of thirty-five observations. The mean of the 

 values obtained for each intensity is given under the heading "Reac- 

 tion time" in Table 11. The rate is equal to . 



Reaction time in seconds 



We have assumed that the rate of movement is an objective meas- 

 ure of the effect of light on the organism, i.e. sensation, and now pro- 

 ceed to consider our experimental results in the light of the Weber- 

 Fechner concept. According to Weber's law, the least noticeable 

 difference of a stimulus is proportional to the magnitude of the pre- 

 existing stimulus. Fechner^ has expressed this relation in the form 



^ Fechner, G. T., Elemente der Psychophysik, Leipsic, 1860, i, 211. 



