PREFACE. 



INTRODUCTORY AND APOLOGETICAL. 



In the spring of 1874 I received from K. B. Tolles, 

 Esq., (the well-known optician of Boston, Mass.,) a 

 one-sixth immersion object glass, which he requested 

 me to study carefully, and to report the results to him. 



The angular aperture of this new one-sixth was said 

 to be 180, and the objective was one of the tirst " du- 

 plex " or four-system glasses devised by Mr. Tolles, and 

 destined, in a few short months, to create quite a stir in 

 microscope circles. 



Having, at that date, leisure at my command, and 

 being very much interested in the performance of ob- 

 ject glasses, I was very glad to give this objective careful 

 and prolonged attention. The result was, that after 

 thirty days' experience I had arrived at a settled con- 

 viction that Mr. Tolles had made a decided advance in 

 the construction and performance of microscope object 

 glasses. 



Believing this experience of mine to be of value to 

 microscopists, I wrote a short account of the perform- 

 ance of this one-sixth, which was published in the Cin- 

 cinnati Medical News. 



Shortly afterwards Mr. Tolles kindly sent me another 

 of the " duplex " or four-system objectives, this time a 



