[cox A callendar] experiments ON THE X-RAYS 187 



the most intense X-radiation which we could produce for upwards of" un 

 hour at a distance of three inches from the tube. Parts of the cultures 

 were shaded from the X-rays by slips of thick lead. The whole of the 

 cultiires were screened from the electrical discharge and from the ultra- 

 violet rays by means of thin aluminium foil. It is not improbable that 

 the neglect of this precaution, which is not generally mentioned, may 

 account for some positive results which have been obtained. The 

 cultures on which we experimented, developed in the normal manner 

 without showing any ti'ace of the action of the X-rays. 



