PREFACE 



TO 

 THE SEVENTH EDITION 



THE use of the Microscope, both as an instrument of scientific research 

 and as a means of affording pleasure and recreative instruction, has 

 become so widespread, and the instrument is now so frequently found 

 in an expensive form capable of yielding in skilled hands good 

 optical results, that it is eminently desirable that a treatise should 

 be within the reach of the student and the tiro alike, which would 

 provide both with the elements of the theory and principles involved 

 in the construction of the instrument itself, the nature of its latest 

 appliances, and the proper comditions on which they can be em- 

 ployed with the best results. Beyond this it should provide an 

 outline of the latest and best modes of preparing, examining, and 

 mounting objects, and glance, with this purpose in view, at what is 

 easily accessible for the requirements of the amateur in the entire 

 organic and inorganic kingdoms. 



This need has been for many years met by this book, and 

 its six preceding editions have been an extremely gratifying evidence 

 of the industry and erudition of its Author. From the beginning 

 it opened the right path, and afforded excellent aid to the earnest 

 amateur and the careful student. 



But the Microscope in its very highest form has become so far 

 at least as objectives of the most perfect construction and greatest 

 useful magnifying power are concerned so common that a much 

 more accurate account of the theoretical basis of the instrument 

 itself and of the optical apparatus employed with it to obtain the 

 best results with * high powers ' is a want very widely felt. 



The advances in the mathematical optics involved in the con- 

 struction of the most perfect form of the present Microscope have 

 been very rapid during the last twenty years ; and the progress in 

 the principles of practical construction and the application of theory 



