IV PREFACE. 



decidedly opposite to those promulgated but a few years 

 ago, that it is evident any one writing on the subject now 

 could only put forth an abstract of the learned Professor's 

 views. This the Author did not wish to attempt, as such a 

 proceeding would only have mystified the student, and not 

 have furnished the advanced worker with as much informa- 

 tion as he could obtain from the 'Journal of the Royal 

 Microscopical Society,' where Professor Abbe's papers may 

 be found in extenso. 



With regard to the selection of objectives, the Author 

 hopes he will not be misunderstood : medium angles have 

 been advised for students' use, for the simple reason that 

 they can be employed without much previous knowledge 

 or difficulty ; but for all purposes of scientific investigation, 

 wide apertures, requiring often much skill in manipulation, 

 will give the most satisfactory results. 



Our American brethren have for many years been im- 

 pressed with the importance of wide apertures, and no 

 doubt many of their arguments were sound, though it can- 

 not be said that their case was ever well demonstrated and 

 supported. 



Dr. G. E. Blackham, in a paper read before the Micro- 

 scopical Congress at Indianapolis in 1878, argued ex- 

 clusively for wide apertures, though he was singularly 

 unfortunate in his selection of a theory to account for 

 "penetration" in objectives; while this brochure has been 

 supplemented by a volume from the pen of Professor J. E. 

 Smith, ' How to See with the Microscope/ putting forward 

 again the merits of wide angles. 



Microscopists who have read these works will do well 



