CORRECTION OF LENSES 



51 



select a definite colour for which this correction is made ; 

 a yellow-green from the brightest part of the spectrum 

 is usually chosen. 



The possibility of the simultaneous elimination of 

 chromatic and spherical aberrations arises from the 

 circumstance that the first is corrected by a careful 

 adjustment of the foci of positive and negative 

 elements whose shapes are immaterial. The second is 

 eliminated by adjusting the shapes of the lenses whose 

 foci have been predetermined. But in the best lenses 

 made from ordinary crown and flint glass the colours, 

 other than those specially selected for correction, still 

 deviate from exact focus, and form the " Secondary 

 Spectrum," focussing together in pairs further away 

 from the lens. Accurate spherical correction, even for 

 the selected colour, is not usually achieved over the 

 entire surface of the lens, and rays from an intermediate 

 zone do not focus strictly with the rays passing through 

 central and peripheral zones. The residual " zonal 

 aberration " is most difficult to overcome. For these 

 reasons the formation of Colour Fringes is still noticeable 

 in the examination of certain objects, particularly when 

 oblique light is used. 



This is necessarily a very incomplete statement of the 

 defects and corrections of lenses, but the outline given 

 is sufficient for an elementary explanation of the differ- 

 ences found in objectives and their bearing on photo- 

 micrography. A discussion of the other difficulties 

 which have to be overcome for instance, those involved 

 in satisfying the " sine law " and preventing "coma " 

 would introduce unnecessary complication, and reference 

 for that should be made to Mr. Conrad Beck's Cantor 

 Lectures, The Theory of the Microscope, to Dr. Spitta's 

 Microscopy, or works on Optics where a fuller survey 

 of the subjects will be found. The adjustment of the 

 different elements that go to make up a corrected lens 



