238 THE AMERICAN MONTHLY [Aug 



The mechanical stage is perfection itself. There is 

 ample room below the stage for all accessories and a 

 swinging mirror bar for the easy examination of urinary 

 tubes and casts. No stand should be made without a 

 swinging mirror bar to give oblique light. In the con- 

 tinental stands there is no room below the stage. If the'y 

 have a condenser it is in a cramped position so that it is 

 almost impossible to get first-class illumination from the 

 mirror. The Jackson arm is preferable to all others be- 

 cause least liable to vibration. These views result from 

 33 years microscopical manipulation and use of the best 

 Spencer and Tolles objectives. Herbert R. Spencer has 

 never had a rival. There have been good and cheap Ger- 

 man objectives but if you wish an objective which will 

 always be free from the dry rot of apochromatic buy the 

 best oil-immersion objectives made by Spencer, or, if you 

 want a dry objective that is perfect, then buy Spencer's 

 1-5 or 1-8 inch of high angle. These have uever had an 

 equal in a long experience uninfluenced by the interests 

 of any firm of manufacturers. 1 have used the Tolles 1-2 

 inch ; 2-3 inch, 6Y° ; 1-6 inch, N. A. 1.37 ; and 1-12 inch ; 

 also, Spencer's 1-4 inch ; 1-15 inch ; 1-25 inch ; Baush & 

 Lomb 1-16 inch ; and Gundlach 1-6 inch 180°. Manipu- 

 lating all these for many years gives reliable knowledge 

 of their relative merits. 



Making Plate and Esmarch Roll Cultures. 



V. A. MOORE. 

 The general principle under-lying the separation of 

 bacteria by means of plate and roll cultures is to dilute 

 the substance containing the bacteria so that the individ- 

 ual organisms will be separated from each other by an 

 appreciable distance and then fixed in a solid medium 

 where each organism can multiply into a growth or col- 

 ony without coming into contact with any other organism 



