CHAP, in.] . THE MICROSCOPE. 237 



different magnifying glasses ; these instruments are then called double 

 and triple lenses. 



To destroy spherical aberration and achromatism at the same time 

 the magnifying glass must be built either of two plano-convex lenses, 

 their convexities facing each other, or of two perfectly achromatic 

 lenses, each formed of two glasses properly chosen, the curves being 

 so calculated as to entirely destroy the spherical aberration. 



Wollaston's periscopic magnifying glass and Brewster's or Cod- 

 dington's lens are on the same principle, that is, the diaphragm is 

 placed in the interior; the glass is a cylindrical sector cut out of a 

 sphere. The middle of the cylinder is grooved, so as to form a 

 diaphragm; a magnifying power of 30 times may be obtained with 

 this lens. 



The Stanhope lens is also formed of a glass cylinder, but the cur- 

 vature of the two surfaces is not the same. By placing small trans- 

 parent objects which are to be examined, such as pollen grains, the 

 scales of butterflies' wings, etc., on the flat surface, and by turning 

 the lens up to the light, bright images are obtained, sometimes mag- 

 nified 40 times. 



II. THE SIMPLE MICROSCOPE WOLLASTON'S DOUBLET. 



The simple microscope (invented by Cuff, and called also Kas- 

 pail's microscope) is a magnifying glass mounted on a brass stand 

 furnished with a stage, on which the object to be examined is placed. 

 Below the stage a plane, or concave mirror, is arranged to throw the 

 light on the object to be examined. By a rack and pinion motion, 

 either the magnifying glass or the stage can be raised or lowered in 

 order to bring the object to a focus that is to say, to place it in the 

 most favourable position for the production of a clear image, a position 

 which varies with individuals and the magnifying powers made use 

 of. The stage is constructed with an opening, which allows the light 

 sent by the mirror to pass, and the object is placed on a glass plate 

 above the opening. 



Fig. 174 represents a more complicated simple microscope. There 

 are two magnifying glasses, which may be inclined, so that all 

 sides of the object may be examined. 



