60 SELECTION AND USE 



class or meeting is large, the best device is that of Prof. O. W. 

 Holmes, whose class microscope, as made by Messrs. J. W. 

 Queen & Co., is shown in Figure 13. This instrument is of the 

 simplest construction, so as to reduce the cost as much as 

 possible, and it is made largely of wood, so as to be light and 

 easily held in the hand. The objects are viewed by direct light, 

 without the intervention of a mirror. The lamp used is a flat- 

 wicked one, the edge of the wick being turned towards the 



Fig. 13. HOLMES' CLASS MICROSCOPE. 



object when high powers are used, and the flat side of the wick 

 when the powers are IOAV. 



" For opaque objects, the lamp is raised to the top of its stand, 

 and its rays allowed to fall upon a small concave mirror at- 

 tacked to the iron frame by a universal joint, whence they are 

 reflected upon the object. The coarse adjustment is effected by 

 sliding the body through a short split tube, and the fine move- 

 ment by means of a delicate screw, which acts on the stage. 

 The entire height of the instrument is about twelve inches, size 

 of base twelve by four inches. 



