VI u PREFACE 



cisms. Professor C. W. Wood worth, author of ''Micro- 

 scope Theory," kindly consented to read part of the final 

 revision. To each of these, the writer's thanks are due. 



Corrections of any of the writer's errors, and new infor- 

 mation from users of the microscope, will be thankfully 

 received. 



This book was written with the permission and assistance 

 of the Carnegie Institution of Washington. 



John Belling. 



Carnegie Institution of Washington, 

 January, 1930. 



