THE MICROSCOPE. 



29 



Optical Improvements in the Simple Microscope. There are 

 imperfections of vision attending the use of all common lenses ; 

 arising either from the shape of the lens, or from the nature of 

 light itself when passing through a refracting medium. These 

 imperfections are termed respectively, spherical and chromatic 

 aberrations. To lessen or destroy these aberrations various 

 plans have been proposed, with various success. Mr. Cod- 

 dington proposed a lens in the form of a sphere, cut away round 

 the centre, as at A, Fig. 6. This is an excellent form for a 

 hand lens, but is not often to be procured in this country, 

 opticians preferring to dispose of the Stanhope lens, seen at B, 



k C} 



which is more easily made than the Coddington lens, but is 

 inferior to it. C and D are doublets proposed by Sir John 

 Herschell; the first of which consists of two plano-convex 

 lenses, a, 5, whose focal lengths are as 2-3 to 1, with their 

 convex sides together; the least convex next the eye, D, 

 consists of a double convex lens, , next the eye, and a meniscus, 

 b. When these lenses are used for forming images the lenses 

 marked a should be next the object. 



Other forms of doublets have been proposed, but by far the 



