PREFACE. 



No apology seems to be necessary for this book. So far as 

 I know, no attempt has been made by any person apparently 

 up in the subject to give instructions for Photo-micrography, 

 since time has elapsed for proper trials of the two great and 

 novel features of the Science, Apochromatic Objectives and 

 Color-correct Plates. My experience, though limited in dura- 

 tion, has been gained by assiduity and very varied practice, 

 and the whole, practically, of my study has been directed to 

 the investigation and utilization of our two new tools above 

 named. 



I have written deliberately and confessedly for the Natural 

 and Medical Sciences rather than for those who use the micro- 

 scope as a pastime. But I have not overlooked the intelligent 

 investigators of such subjects as diatom-structure and the like ; 

 for in objects whose real structure is so problematic lies not 

 only a valuable field for optical research, but much of the 

 future of practical optics, and perhaps the very best manege 

 for the student photo-micrograph er. I boast, however, to 

 have written something that will materially help those who 

 study the Natural Sciences in general rather than Optical 

 Science in particular. 



I have to thank many friends for help of various kinds. 

 Mr. Edward M. Nelson, my instructor in practical microscopy, 

 has placed me under the most marked obligation, for he kindly 

 read my M.S. so far as it touched optical theories, and he 

 directed me where necessary. I owe so much generally to 



