president's address. 205 



Doublet of no aberration. Refractive index = 1*50 

 1st lens, crossed r = + 291 



s = — 1750 /=+ -5 

 2nd lens, bi-concave r = — '683 



s = + 1-865 /=— 1*0 

 Focus of the combination = + 1*0. Aberration = 0. 



New doublet of minimum aberration. Refractive index 

 = 1-50 



1st lens, crossed r = + 1'166 



s = — 7-000 / = + 2-0 

 2nd lens, meniscus r = + '636 



s = + 1-750 /= + 2-0 

 Focus of combination = +1-0 



2/ 2 



Aberration = — -214 — 



F. 



Best form of chromatic doublet for a microscope Bull's eye. 

 Boro-Silicate crown. Jena Catalogue O. 144. Refractive 

 index 1*51 



1st lens, crossed r = + 2"359 



s = — 15-078 diameter 2*1 



2nd lens, meniscus r = + 1*280 



s = + 3*434 diameter 1*8 



Distance between lenses '05 ; equivalent focus 2*0 ; back 

 focus 1*55; total aberration — -103; clear aperture 2'0 ; angle 

 62°. Note the second Gauss point of the combination is close 

 to the posterior surface of the crossed lens. 



This combination is eminently suitable for photomicrography 

 in those cases where a Bull's eye is necessary. 



