Ch. XII] 



HISTORY OF LENSES AND MICROSCOPES 



429 



their easier handling or focusing. The common reading glass with 

 its convenient handle (fig. 4) and the tripod (fig. 201) and focusing 

 lens holder (fig. 202) are good examples. 



In reading the older literature one often meets with the expression 

 " single microscope." This means a simple microscope, composed of 

 one lens (fig. 182), and is in contrast with the " double microscope," 

 or compound microscope of two lenses or two combinations (objective 

 and ocular, fig. 248-249). 



«** 



Ocular 



'-y-y-yy 



Fig. 248, 249. Dutch and Keplerian Compound Microscopes for 



Comparison. 



Each has a convex lens for objective. For ocular the Dutch form has a concave 

 and the Keplerian form a convex lens. The ocular for the Keplerian form is 

 properly a magnifier of the real image, while the concave-lens ocular of the Dutch 

 microscope acts as an amplifier for the objective. 



The virtual image is erect with the Dutch, but inverted with the Keplerian 

 microscope. 



§ 695. The Dutch compound microscope. — So far as known at 

 present the first compound microscope invented was composed of 

 two lenses, a convex lens for the objective and a concave lens for the 



