OBSERVATIONS ON UNICELLULAR FORMS 135 



The wave length of the rays used in Hertel's experiment 

 was only 280 fxij,, while the length of the shortest wave of 

 the visible spectrum is a little over 400 ^t//. Paramecia 

 continuously exposed to the ultra-violet rays are injured 

 almost immediately; their movements become uncoordi- 

 nated and they die in from 10 to 50 seconds. It seems 

 probable, then, that these rays stimulate Paramecia because 

 of their injurious effect. 



