Chap. XXVI II.] ACHR OMA TIC OBJECTIVE. 361 



objective. The object on the stage of the microscope 

 is often covered with fluid, and a cover-glass. Rays 

 from the object are dispersed to some extent in pass- 

 ing through the film of liquid or the cover, and if the 

 magnifying power employed be very high, chroma- 

 tism results. This may be corrected by altering 

 the position of the lenses in the object-glass. Hoss, 

 of London, therefore, constructed an objective as 

 shown in Fig. 162, such that the lens next the object 

 was placed in the tube , while the other two were 

 fixed in the tube b. A screw at the side permits 

 the lowermost lens to be moved nearer to, or farther 

 away from, the other two, and so the lens can be 

 adjusted for viewing an uncovered or a covered 

 object. 



The general principles that have been explained 

 are those applied in the construction of the best 

 modern microscopes. Lenses made of crown and flint 

 glass are used and combined into sets. The method 

 of combination varies, however. Thus, instead of 

 three lenses, each of which is a doublet (i.e. made of 

 two lenses cemented together), in one arrangement 

 the middle lens is a triplet, consisting of a doubly 

 concave lens of flint between two convex lenses of 

 crown glass, the other two being plano-convex lenses 

 of crown glass. In another combination the back lens 

 is a triplet, the middle one a doublet, and the front 

 one a single plano-convex lens. 



Now supposing an object-glass is obtained cor- 

 rected for spherical and chromatic aberration, it is 

 evident that, if the eye-piece is chromatic, blurred 

 and coloured images will still be obtained, though 



O o 



to a less extent. The eye-piece must be achromatic 

 as well as the object-glass. An eye-piece devised 

 by Huyghens for getting rid of spherical aber- 

 ration in the eye-piece of telescopes is found to 

 answer the purpose, and to be not only free from 



