72 



THE MICROSCOPE. 



concave lens of flint glass between them, the intermediate 

 combination being a doublet composed of a double-convex 

 lens of crown glass with a double-concave of flint glass ; 

 and the front combination being another triplet composed 

 of two plano-convex lenses of crown, and a plano-concave 

 lens of flint glass between them. This will also explain 

 the composition of the shallower object-glasses ; the dis- 

 tance between the different combinations throughout all 

 the objectives, and their size, depending on the amount of 

 magnifying power required. It may be briefly stated that 

 the principal qualities to be looked for in an object-glass 



^^ 



^ 





! 



Fig. 38. — Forms of Object-glasses. 



A, Double-convex lens ; B, Plano-concave ; C, Bi-convex and plano-concave, 

 united : shown in their various combinations, as at D, form the 3-in. 2-in. or 

 lj-in. ; at E, 1-in. and f-in. ; and at F, the |-in. fo-ia. i-in. and ^-in. ob- 

 jectives. 



are, 1st, Resolution, or the power of showing clearly minute 

 details, wliich is chiefly effected by increase of light ad- 

 mitted through the objective, by what is termed angular 

 aperture ; 2d, Penetration, or that power which enables 

 the observer to see deep into the structure of objects with- 

 out any alteration of focus ; 3d, Definition, or the capa- 

 bilities of an objective for showing the various details of 

 an object, especially its boundaries, with the most perfect 

 and exquisite sharpness ; 4th, Flatness of field, or marginal 

 and central definition, which denotes the exact capability 



