THE MICROSCOPE. 



CHAPTEE II. 



Mutual chromatic and spherical correction of the lenses (Continued). 

 18. Centering. 19. Compound object-pieces. 20. The eye-piece. 

 21. Various magnifying powers adapted to the same microscope. 

 22. Actual dimensions of the field of view. 23. Means of moving 

 and illuminating the object. 24. Focussing. 25. Preparation of the 

 object. 26. General description of the structure of a microscope. 

 27. The stage. 28. The illuminators. 29. The diaphragms. 



Now let us suppose that the entire combination of lenses is so 

 formed, that the negative chromatic aberration produced by the 

 object-glass shall be exactly equal to the positive chromatic 

 aberration, produced by the field-glass and the eye-glass. In 

 that case, it is evident that the one aberration will extinguish the 

 other, and that the image viewed by the observer through the eye- 

 glass will be an exact reproduction of the object, being exempt 

 from all aberration whatever. 



To make this more evident, Let LL, fig. 10, be the compound 

 object-glass, consisting of a double convex lens of crown glass, 

 and a plano-concave lens of flint-glass, formed so as to produce 

 negative chromatic aberration ; let F F be the field-glass, E E the 

 eye-glass, and o the object. 



Let v v R E, be the coloured images of the objects, which would 

 be produced by L L, if F F were not interposed ; these images will 

 be slightly concave towards L L, according to what has been 

 explained in our Tract upon ' ' Optical Images," 47 et seq. ; and since 

 L L is supposed to have negative aberration, the red images n R 

 will be nearest to it, and the violet ones, v v, most remote from it. 



But the rays which would converge upon the various points of 

 these images being intercepted by the field-glass F F, are rendered 

 more convergent by it, and the images are accordingly formed 

 nearer to it. This lens, F F, also increases the convergence of the 

 violet rays which are most refrangible, more than it increases that of 

 the red rays which are least refrangible. The consequence of this 

 is, that the violet and red images are brought closer together than 

 they were, as shown in the figure ; but still the violet images are 

 more distant from F F than the red, so that the chromatic aberra- 

 tion of L L and F F conjointly is still negative, though less than 

 the aberration of L L alone. 



There is another effect produced by the lens F F which it is 

 important to notice. The images produced by L L, which were 

 slightly concave towards F F, are changed in their form, so as to 

 be slightly concave towards E E. The cause of this change has 

 been already explained in our Tract upon u Optical Images," 46. 

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