592 WHEELER P. DAVEY 



3. It has been shown that the lethal effect of X-rays on Tri- 

 bolium confusum bears a definite mathematical relation to the 

 logarithm of the total X-ray dose. 



4. An extension of the Psycho-physic law gives a theoretical 

 explanation of the experimental data, if the resistance rather than 

 the susceptibility of the organism to the X-rays is considered. 



The effects produced by changes in voltage, and by dividing 

 the dose of X-rays into small parts, will be taken up in a later 

 paper. 



