40 



HANDBOOK OF PHOTOGRAPHY 



ably thin cemented doublet, the ratio of the V of each glass ^ to its refractive index n 

 must be the same. Unfortunateh^ in ordinary crown and flint glasses the n and V 

 change in opposite directions, as indicated in Table I. 



Table I. — Index and Dispersion of Old Types of Glass 



However, by the use of barium glasses introduced by Schott and Abbe in 1886, 

 it becomes possible to select pairs of glasses in which the values of V/n are the same 

 (Table II). 



Table II. — Index and Dispersion of Some "New" Glass Pairs 



The first lens to include these new glass pairs was the Ross Concentric (Fig. 9) 

 designed by Schroder in 1888. This lens was a symmetrical system of two flat-field 

 achromatic landscape lenses; not being spherically corrected, its aperture was limited 

 to //16, but it satisfactorily covered a wide field. 



Now to correct the spherical aberration of a thin cemented doublet it is necessary 

 that the convex element should have a lower index than the concave, as in the Rapid 



I 



Fig. 9. — Ross Concen- 

 tric lens. 



Fig. 10.^ — Protar 

 lens developed by 

 Rudolph. 



Fig. 11. 



— Protar //4. 5 

 lens. 



Rectilinear type, but in a new achromat combination the convex element must have a 

 higher index than the concave. It is thus impossible to design a thin spherically 

 corrected achromatic lens of the new glasses. Dr. Rudolph of the Zeiss Company 

 realized this, and in 1890 produced his Protar lens (Fig. 10) consisting of an old-type 

 achromat in front, and a new achromat behind. The front lens was given a verj'^ low 

 or zero focal power, and it had enough overcorrected spherical aberration to com- 



' The V-number of a glass is the reciprocal of its dispersive power, or F = («d — l)/{nF — nc). It 

 is sometimes called the "constringence" or Abbe number of the glass. 



