THE MICROSCOPE. 23 



and Mr. Codclington invented a lens in the form of a 

 sphere, cut away round the centre so as to assume the 

 shape of an hour-glass. This latter, in a convenient set- 

 ting, is one of the best pocket microscopes. Dr. \Vollas- 

 ton's doublet consists of two plano-convex lenses, whose 

 focal lengths are as 1 to 3, with the plane sides of each 

 and the smallest lens next the object. They should be 



FIG. 1. 



Holland's Triplet. 



about the difference of their focal lengths apart, and a 

 diaphragm or stop an opaque screen with a hole in it 

 placed just behind the anterior lens. This performs ad- 

 mirably, yet has been further improved by Mr. Holland 

 by making a triplet of plano-convex lenses (Fig. 1), with 

 the stop between the upper lenses. 



The Compound Microscope consists essentially of two 

 convex lenses, placed some distance apart, so that the 

 image made by one may be magnified by the other. 

 These are called the object-glass and the eye-glass. In 

 Fig. 2, A is the object-glass, which forms a magnified 

 image at c, which is further enlarged by the eye-glass B. 

 An additional lens, D, is usually added, to enlarge the 

 field of view. This is called the field-glass. Its office, as 

 in the figure, is to collect more of the rays from the 

 object-glass and form an image at F, which is viewed by 

 the eye-glass. 



Owing to chromatic aberration, an instrument ot this 

 kind is still imperfect, presenting rings of color round the 

 edge of the field of view as well as at the edge of the 

 magnified image of an object, together with dimness and 



