OF MICRO-ORGANISMS. 89 



DelbcEuf has happily formulated as follows: " The 

 slightest difference capable of being felt between two 

 excitations of the same sort is due to an actual differ- 

 ence that increases proportionally with the excitations 

 themselves." Thus, in order to tell that one weight is 

 heavier than another, it must be heavier than the 

 other by a fractional difference which varies from one 

 third to one fifth according to the individual, be the 

 original weight what it may. For example, to a weight 

 of three grammes, in order that a difference may be 

 made perceptible, we must add one third of three 

 grammes or one gramme. To four grammes we must 

 add one third of four grammes, or one and one third 

 grammes, etc.* 



According to Pfeffer, the application of Weber's law 

 to his experiments is so exact that, when the solution 

 of the tube is only twenty times stronger than that of 

 the watch-crystal, the spermatozoids remain unaf- 

 fected. Furthermore, the application of the law is not 

 disturbed by changes of temperature varying within 

 certain limits. Thus, down to a temperature of -|- 5° 

 (41° Fahr.) the spermatozoids remain sensible to a 

 concentratipn of liquid thirty times as strong as that 

 in which they are. 



Basing his calculations upon these experiments, 

 the author has succeeded in determining the probable 

 quantity of malic acid that must be contained in the 

 archegonium. This quantity is probably in the pro- 

 portion of three tenths. 



The spermatozoids of the Selaginella are likewise 

 wise attracted by malic acid and the malates. As re- 

 gards the Marciliaceae, the specific substance has not 

 been discovered. The same failure, also, in the case 



♦Consult Ribot, Psyckologie allemande, p. l6l. 



