52 PHOTOMICROGRAPHY 



is a mathematical problem of great delicacy, involving 

 a very careful selection of the materials and an accurate 

 computation of their curvatures, powers, and shapes, so 

 as to eliminate all the errors simultaneously, and full 

 correction is only attained by the combination of as many 

 as six or even ten lenses in one objective. 



Objectives. A lens corrected for spherical error is called 

 "Aplanatic" and for chromatic error, "Achromatic." The 

 objectives generally used are divided into two classes, 

 according to the way in which these corrections are carried 

 out, called respectively, "Achromatic" and "Apochromatic." 

 Messrs. Zeiss have recently added a third group for use with 

 ultra-violet light, to which they give the name " Mono- 

 chromatic." The newer types of Achromatic objectives 

 are such an advance over their predecessors that they 

 are almost worthy of classification in a special group 

 between the Achromatic and Apochromatic and, in fact, 

 several makers call them " Semi- Apochromatic." 



Achromatic Objectives are chromatically corrected in 

 such a way that two colours come to the same focus, 

 while the most accurate spherical correction is confined 

 to rays of one selected colour. They show more or less 

 zonal aberration. 



As these objectives are primarily intended for visual 

 work, the colour selected for the most accurate spherical 

 correction is usually a yellowish green from the most 

 luminous part of the spectrum. If matters are so 

 arranged that the other images are practically coinci- 

 dent with that formed by the preferred colour, the lens 

 will give very good photomicrographs, and good modern 

 achromats fulfil this condition very well, but an abso- 

 lutely sharp negative cannot be obtained when using 

 white light, although the error is frequently small enough 

 to be unimportant except for the most exacting work ; 

 on the other hand, many older lenses are very deficient 

 in this respect, unless they were specially corrected for 



