262 



THE SIMPLE MICKOSCOPE. 



the simple Microscope. A description of the devices of the past 

 century lies beyond our sphere ; we must refer our readers to the 

 copious work of Harting in regard to this point and for the history 

 of the Microscope. 



Brewster was the first to hit upon the happy idea of making a 

 mngnifying-glass by grinding a tolerably deep groove in a glass 

 sphere (Fig. 142), which gives images approximately free from 

 aberration. Coddington facilitated their production by giving to 

 the groove an angular form (Fig. 143), making the two halves of 



FIG. 142. 



FIG. 143. 



FIG. 144. 



the sphere separately out of suitable pieces of plano-convex lenses, 

 which were afterwards cemented together at their plane surfaces. 

 The shape of the groove is of no consequence, and it is obvious- 

 that the construction represented in Fig. 144 answers the pur- 

 pose in view equally well. These Coddington lenses (also called 

 Coneopsids or Bird's-eye lenses), as manufactured by Lerebours, of 

 Paris, met with general approval ; according to Mohl, they give an 

 amplification of upwards of 20 linear, and he commends them 

 highly. They are, however, defective in two ways, which are 

 important in many researches : the field of view is very limited, 

 and the distance of the focus from the lower surface is only about 

 J of the diameter of the sphere. When, therefore, it is required to 

 observe an object of rather large dimensions, which is the most 

 frequent case in practice, the modern aplanatic com- 

 binations of lenses are preferable. 



With those just mentioned are closely connected 

 the cylinder magnifying- glasses, which have long been 

 used, especially in Germany. They consist of a 

 cylindrical piece of glass, the two terminal surfaces of 

 > 14 - which are ground convex, of unequal curvatures to 

 diminish the aberration (Fig. 145). The shallower 

 curvature is usually directed to the object. The cylinder lenses 

 produce somewhat less perfect images than the Coddington, and 



