288 



CORRECTION OF ABERRATIONS 



[Ch. IX 



§ 462. Table showing cause of spherical aberration in the 

 microscope and means of correction. — 



Under-correction produced by: 



Too weak a concave element in the 

 objective. 



Too close an approximation of the 

 lenses of the objective. 



Too short a tube, that is the ocular 

 and objective are too close to- 

 gether. 



Use of too thin a cover-glass. 



Over-correction produced by: 



i a. Too strong a concave element in 



the objective. 

 2a. Too great a separation of the 



lenses of the objective. 

 3a. Too long a tube, that is the ocular 



and objective are too far apart. 



4a. Use of too thick a cover-glass. 



Any defect can be neutralized by applying the right amount of 

 what would produce the opposite condition. For example, the over- 

 correction produced by too thick a cover-glass can be corrected by: 

 (4) Using a thinner cover-glass; (3) Shortening the tube; (2) Putting 

 the lenses of the objective closer together; (1) using a weaker con- 

 cave element in the objective. 



If there is under-correction from too short a tube it can be neutral- 

 ized by: (3a) lengthening the tube; (4a) using a thicker cover-glass; 

 (2a) separation of the lenses of the objective; (ia) using a stronger 

 concave element in the objective. And similarly with under-correction 

 or over-correction from any cause; opposites neutralize. 



§ 463. Chromatic aberration. — Spherical aberration which has 

 just been discussed is present in lenses even when the light is of one 

 wave length; chromatic aberration, on the other hand, appears in 

 addition when composite light traverses a lens. This is because 

 every wave length of necessity is differently refracted; the shortest 

 waves most, the longest waves least. If then a single beam of white 

 light traverses a lens, the different wave lengths will be refracted differ- 

 ently and the blue-violet waves made to cross the axis first, the red 

 waves last. There will be then a series of colored foci extending along 

 the axis, as shown in fig. 171. Every simple lens, then, whose aperture 

 is filled with composite light will show both spherical and chromatic 

 aberration, and the greater the aperture and the shorter the focus 

 the more pronounced will be both forms of aberration. In order that 

 perfect images may be produced, both aberrations must be eliminated. 



