PREFACE TO THE SEVENTH EDITION IX 



words which show his cordial friendliness, he says : ' I find the whole 

 . . . much more adequate to the purposes of the book than I should 

 have been able to write it. ... I feel the greatest satisfaction in 

 seeing my views represented in the book so extensively and inten- 

 sively.' 



These words are more than generous ; but I quote them here 

 in order that the reader may be assured of the accuracy and 

 efficiency of the account given in the following pages of the invalu- 

 able demonstrations, theories, and explanations presented by Dr. 

 ABBE on the optical principles and practice upon which the recent 

 improvement in the construction of microscopical lens systems has 

 so much depended. 



It will not be supposed that I implicitly coincide with every 

 detail. Dr. ABBE is too sincere a lover of independent judgment 

 to even desire this. But it was important that his views as such 

 should be found in an accessible English form ; in that form I have 

 endeavoured to present them ; and in the main there can be no 

 doubt whatever that these teachings are absolutely incident with 

 fact and experience. In details, as may appear here and there in 

 these pages, especially where it becomes a question of practice, I may 

 differ as to method, and even interpretation, from this distinguished 

 master in Mathematical Optics. But our differences in no way affect 

 the great principles he has enunciated or the comprehensive theory 

 of microscopical vision he has with such keen insight laid down. 



In preparing the remainder of the seven new chapters of this 

 book I have sought and, without hesitancy, obtained advice and 

 the advantage of the support of my own judgment and experience 

 from many competent men of science, who have shown a sincere 

 interest in my work and have aided me in my endeavours. But 

 first on the list I must place my friend Mr. E. M. NELSON. Our 

 lines of experience with the Microscope have run parallel for many 

 years, although the subjects of our study have been wholly different ; 

 but the advantages of his suggestion, confirmation, and help have 

 been of constant and inestimable value to me. He placed his know- 

 ledge, instruments, and experience at my disposal, fully and without 

 limit or condition ; and his exceptional skill in Photo-micrography 

 has enabled me to add much to the value of this book. 



To Count CASTRACANE I am indebted for valuable suggestions 

 regarding the Diatomacea?, to lie used at my discretion ; to Dr. 

 VAN HEURCK I am also under much obligation for his courtesy in 

 preparing Plate XI. of this book, giving some of his photo-micro- 

 graphic work with the new object-glass of 2 - 5 mm. N.A. 1*60. 

 The full description of this plate is given, with some critical remarks, 

 in the General Description of Plates. To the late and deeply 



