The author wishes to acknowledge his gratitude to Mr. L. de 

 Ruiter, Groningen, who undertook the task of translating the 

 book and preparing the manuscript of the English edition. He 

 also sincerely thanks Prof. J. E. Harris, of Bristol University, 

 for reading the manuscript and for helpful suggestions and 

 constructive criticisms. 



Chr. P. R. 



Translator's Note 



I am greatly indebted to Prof. J. E. Harris, who carefully 

 read the whole manuscript of the translation, and suggested 

 many improvements in style and choice of terms. 



L. de R. 



