68 BAC?rEEIOLOGY. 



The chromatic aberration may be reduced by stopping out the 

 marginal rays ; but as it is necessary to get the most perfect 

 correction possible, advantage is taken of the different relations 

 which the refractive and dispersive powers bear to each other in 

 diflferent glasses. By combining a double convex lens of crown 

 glass with a plano-convex lens of flint glass, correction is obtained 

 for the violet and red rays. An achromatic objective is constructed 

 on this principle, but the result is not perfect, as the intermediate 

 coloured rays remain uncorrected, and what is termed a secondary 

 spectrum gives rise to images with coloured fringes, especially at the 

 margin of the field. Abbe and Schott, after a great number of 

 experiments, succeeded in discovering a glass with optical properties 

 which removed the secondary spectrum, and objectives made with 

 the new glass are termed apo-chroTnatic. There is much more 



Fig. 14. — Chkomatic Abekbation. 



perfect concentration of the component rays than in the ordinary 

 achromatic objectives, and the advantages thus obtained are very 

 great. The objectives can be made of higher angle and admit of 

 higher eye-pieces being used without materially diminishing the 

 brilliancy and definition of the image. There is a complete absence 

 of coloured fringes, and the perfect definition is invaluable in 

 micro-photography. 



Another fault which has to be corrected is the aberration caused 

 by covering a microscopical preparation with a cover-glass. Ross 

 was the first to point out the difference in the image when the 

 object was examined under a cover-glass, and that by altering the 

 position of the front pair of lenses, in an objective corrected for an 

 uncovered object, the objective could be corrected for the covered 

 object (Fig. 15). 



Objectives are generally corrected for a standard thickness of 

 cover-glass, but H. Lister devised a screw-collar adjustment by 

 which the position of the front pair of lenses could be altered at 

 wUl ; and as it is almost impossiblfe to obtain cover-glasses which 



