PREFACE. vii 



present state of science as regards the minute organ- 

 isms of which it treats. No doubt the book contains 

 much that will not bear rigid scientific crijticism ; and 

 the constant additions to our knowledge which are 

 daily being made will necessitate frequent revisions and 

 additions, if a favorable reception by the Medical Pro- 

 fession, and students of Biology, makes it practicable 

 for the writer to carry out his present intention of rep- 

 resenting in future editions the progress which may be 

 made in the interval. I am not prepared to say, how- 

 ever, that the heliotype plates which illustrate this 

 edition will appear in subsequent editions, if they are 

 called for. These plates add greatly to the cost of the 

 volume, and they will perhaps be less satisfactory than 

 lithographs or wood-cuts to those not accustomed to 

 similar views under the microscope, and to those critics 

 who are not familiar with the technical difficulties 

 attending an attempt to photograph the minute organ- 

 isms here represented. If the clean field and sharply- 

 drawn outlines which it is so easy to draw upon wood 

 or stone makes a prettier picture, and one which may 

 be preferred by some, there can be no doubt that these 

 views from nature, if closely studied, are more instruc- 

 tive than drawings, notwithstanding the inevitable de- 

 fects arising in some instances from the presence in the 

 field of view of extraneous objects, and from the im- 

 possibility of having every part of the field in the best 

 possible focus at the same time in these photo-micro- 

 graphs, which are made with objectives of high power 

 having an extremely limited focal range. 



G. M. S. 



FORT MASON, SAN FRANCISCO, 

 August 15, 1883. 



