Vlll PREFACE 



and that Mrs. Dolores Johnson, and Miss Kristine Pallesen, have stood by 

 the author durmg the distressing hurly-burly known as "getting the book 

 into press." 



Dr. James Lackey, then scientific editor of the Blakiston Company, encour- 

 aged me over long periods to persevere in producing a publishable book. 

 His successor, Mr. WiUiam Keller, approved what had been done and, with 

 Mr. Willard Shoener converted my efforts to their present form. My debt to 

 these gentlemen, and to their editorial assistants, is immense. 



Acknowledgement is made with thanks to the American Optical Company 

 for figures 56, 57, and 84, to the Fisher Scientific Company for figures 34 and 

 38, and to the Carbide and Chemical Corporation for some of the data in 

 Chapter 25. Permission to reproduce copyright material of the R. R. Bowker 

 Company and the Oxford University Press is specifically acknowledged at the 

 places where these reproductions occur. 



Peter Gray 

 Edinburgh 1933 

 Pittsburgh 1953 



