PREFACE. VU 



by a 4-10ths, a l-4th, or even a l-8th, with deep eye-pieces; but that Mr. 

 D. is satisfied that if he had tried to do the work of the last ten years on 

 the latter plan, " he would be now blind, instead of possessing as good 

 and sensitive a sight as he had ten years ago." As it has been politely 

 suggested by an American controversialist, that the Author's inability to- 

 appreciate the supreme value of wide aperture may be due to the senile 

 deterioration of his vision, the Author ^ s na PPJ to be able to state that, 

 thanks to his habit of using shallow Eye-pieces, and of never persisting 

 in Microscope-work when he has felt visual fatigue, his eyes are now as 

 fit for Microscopy as they were when he began so to use them nearly 

 half a century ago. 



He has only to add that he has endeavored, by a careful and thorough 

 revision of the entire Treatise, to render it as serviceable as possible to- 

 those for whom it is specially intended. Besides introducing a large 

 amount of new matter into the first four chapters, he has entirely re- 

 written Chap. V., so as to embody in it an account of those methods of 

 Hardening, Staining, Imbedding, and Section- cutting, which have com- 

 pletely revolutionized many departments of Microscopic investigation. 

 In the sections relating to the Protophytic forms of Vegetable life, much 

 new matter has been introduced in regard so the Scliizomycetes or Bacte- 

 rium group, the Myxomycetes, and others of those curious organisms- 

 which occupy the border-ground between Vegetable and Animal life. 

 So, again, in the section on the Protozoic forms of Animal life, large ad- 

 ditions have been made under the heads of Monerozoa, Rhizopoda, Infu- 

 soria (especially i\\Q flagellate &&& suctorial), and Radiolaria; and the sec- 

 tion on Sponges has been entirely re-written. Some important additions 

 have also been made (Chap. XXI. ) in regard to the applications of the 

 Microscope to Geological inquiry. In many other instances, references 

 have been made to the best sources of information upon recent discov- 

 eries of interest, which a due regard to the necessary limits of his book 

 made it requisite for the Author to dismiss with a mere mention. 



No fewer than fifty new Wood-engravings have been added (for the 

 use of eleven of which the Author is indebted to the Council of the Lin- 

 naean Society), besides the reproduction of Prof. Cohn's beautiful Plate 

 of Volvox, which now forms the Frontispiece. 



To such as feel inclined to take up the use of the Microscope as a 

 means of healthful and improving occupation for their unemployed hours, 

 the Author would offer this word of encouragement, that, notwithstand- 

 ing the number of recruits continually being added to the vast army of 

 Microscopists, and the rapid extension of its conquests, the inexhaustibility 

 of Nature is constantly becoming more and more apparent; so that no 

 apprehension need arise that the Microscopist's researches can ever be 

 brought to a stand for want of an object! 



LONDON, May, 1881. 



