THE MICROSCOPE. 



in fine, then, the rays diverging from the images R' n' v' V, 

 after passing through the eye-glass E E, have their divergence 

 diminished, so as to diverge from more distant points, u. The 

 divergence of the violet rays,v' V, being most refrangible, is most 

 diminished, and that of the red rays, E' n', being least refrangible, 

 is least diminished. Ittheir divergence were equally diminished, 

 a series of coloured images would be formed at 1 1, the violet being 

 nearer to, and the red farther from E E ; but the divergence of the 

 violet, which is already greater than the red, is just so much 

 greater than the latter, that the difference of the effects of E E 

 upon it is such as to bring the images together at 1 1. 



Thus it appears, that the positive aberration of the eye-glass 

 E E is exactly equal to the negative aberration of L L and r F taken 

 conjointly, so that the one exactly neutralises the other, all the 

 coloured images coalescing at 1 1, and producing an image alto- 

 gether exempt from chromatic aberration. 



There is another important effect produced by the eye-glass ; 

 the images n' n' v' V, which are slightly concave towards E E, 

 are rendered straight and flat at 1 1 ; the principle upon which 

 this change depends has been also explained in our Tract upon 

 " Optical Images," 46. 



Thus, it appears that, by this masterly combination, a multi- 

 plicity of defects, chromatic, spherical, and distortive, are made, 

 so to speak, to extinguish each other, and to give a result, 

 practically speaking, exempt from all optical imperfection. 



18. There is still another source of inaccuracy which, though it 

 is more mechanical than optical, demands a passing notice. All 

 the lenses composing the microscope require to be set in their 

 respective tubes, so that their several axes shall be directed in 

 the same straight line with the greatest mathematical precision. 

 This is what is called CENTERING the lenses, and it is a process, 

 in the case of microscopes, which demands the most masterly skill 

 on the part of the workman. The slightest deviation from true 

 centering would cause the images produced by the different lenses 

 to be laterally displaced, one being thrown more or less to the 

 right and the other to the left, or one upwards and the other 

 downwards ; and even though the aberrations should be perfectly 

 effaced, the superposition of such displaced images would effec- 

 tually destroy the efficiency of the instrument. 



19. In what precedes, we have, to simplify the explanation, 

 supposed the object-glass to consist of a single achromatic lens, a 

 circumstance which never takes place except when very low 

 powers are sufficient. A single lens, having a very high magni- 

 fying power, would have so short a focus and such great curvature, 

 that it would be attended with great spherical aberration, inde- 



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