62 



THEORY OF THE MICROSCOPE. 



piece) different objectives must be employed. 1 In the higher 

 powers the front lens is generally mounted, so that its distance 

 from the others can be regulated to allow for the influence of 

 the cover-glass or the obliquity of the incident rays. 



The correction of the aberrations is therefore effected in practice, 

 as in theory, by the method of approximation. In practice, the 

 testing begins with the last refractions in the system, and proceeds 

 downwards to the primary one ; in theory, the ray of light is 

 traced from the anterior surface of the objective up to its last 

 surface. 



Special attention is due to the effects of spherical aberration 

 in the eye-piece. Since the cones of light which reach the points 

 of the objective-image are much diminished, and meet only an 



FIG. 21. 



exceedingly small portion of the field-lens, it is not single rays 

 which experience the stronger refraction of the margin of the 

 lens, as in the objective, but the whole image-forming pencils 

 which pass through this margin. Spherical aberration is not 

 exhibited through the obliteration of single image-points, for the 

 aberrations within a pencil of rays are hardly perceptible ; it 

 influences, however, the direction of the axes of the pencils after 

 refraction, and hence the relative position of the image-points. 

 As a demonstration of this action, let E* in Fig. '21 be the second 

 principal point of the objective, in which the axes of the imago- 



1 The high-powers of Hartnack, Beneche, Kellner, Plo-ssl, &c., are con- 

 structed on this principle. The two posterior lenses alone give a very indistinct 

 image, which is over-corrected both spherically and chromatically. The blue 

 mist surrounding the outlines is generally so considerable, that it might be a 

 priori doubted whether so much aberration could be compensated in a satis- 

 factory manner by the addition of the anterior lens. It cannot, however, be 

 disputed that in practical Optics the best results have hitherto been obtained 

 with such combinations of lenses. 



