381 



Calculation of Thresholds and Velocities. 



a. T/ircslioIds. Krogh ('14) showed that the zero of the equilateral 

 hyperbola to which the time-temperature curve partly conforms, is not 

 the actual threshold of development. Values calculated for those parts 

 of the time-temperature curve which conform to the equilateral hyperbola 

 (within the straight-line limits of the velocit)' curve) do have a signifi- 

 cant relation to the actual limits, however, and correct methods of obtain- 

 ing them are important. 



In Table XV are shown the results obtained from a simple formula 

 in calculating alpha values for high-humidity and low-humidity experi- 

 ments of 1917. The humidities above 69% and below 61% have been 

 grouped separately, and the data here serve chiefly to bring out the fact 

 that the calculated alplia is lower in the high-humidity experiments than 

 in the low-humidity experiments and also that it is lower in the summer 

 generation than in the spring generation. 



Table XVI. i>hoiving the use of Von Oettingen's phenologieal method of deter- 

 mining the alpha value* 

 (See Pig. 13, curves for 95% humidity.) 



• Only results at the same temperature and humidity were averaged together. 

 With a weighted average, the alpha value is 54.8° F. 

 t With 199.5 omitted, the mean is 263. 



