viii Foreword to the First British Edition 



Royal Society of London; and to the following firms : General 

 Electric Co. (Schenectady), General Electric Co. (Wembley), 

 Dow Chemical Co., B. F. Goodrich Co., Eastman Kodak 

 Research Laboratories, and the Radio Corporation of America. 

 Special thanks are due to Dr. V. K. Zworykin, Associate Di- 

 rector of the R.C.A. Research Laboratories, for the kind loan 

 of several original micrographs and other material. 



The author thanks the Directors of the British Thomson- 

 Houston Co. for permission to publish this monograph. 



