114 Tue MIcroscoPe. 
chemical elements, O, Ca, K, Na, Pb, and Si, whilst the new ob- 
jective contains not less than fourteen elements. 
The optical principle on which the objectives have been con- 
structed is indicated in a paper by Prof. Abbe in this Journal. 
‘*On new methods for improving spherical correction,” &c. In 
fact, all the work of Prof. Abbe and Dr. Schott during the five 
years has been solely directed to finding the proper means for 
the realization of the desideratum there mentioned, viz. doing 
away with the secondary chromatic aberration, and with the 
chromatic difference of spherical aberration. The proper means 
was found in special kinds of glass, which allowed of propor- 
tional dispersions in different parts ot the spectrum, and which 
at the same time exhibit different relations between the refrac- 
tive indices and dispersive powers. By these means a more 
perfect concentration of all the rays emanating from the object 
is obtained. With the old kinds of crown and flint glass tee 
different colours only could be collected to one focus, a second- 
ary spectrum remaining uncorrected, whilst the new objectives 
collect three rays of different colors to one focus, leaving a 
small tertiary spectrum only. Moreover, spherical correction 
has hitherto been confined to rays of one color, being made for 
the central part of the spectrum, the objective remaining wnder- 
corrected spherically for the red rays and over-corrected for the 
blue rays. In the new objectives, however, the correction of 
the spherical aberration is obtained for ¢wo different rays of the 
spectrum, that is practically for all colours at the same time, 
and the objective shows the same degree of chromatic correc- 
tion for the central as for the marginal part of the aperture. All 
this requires greater complication in the construction, hence the 
use of five lenses instead of the four hitherto employed. In ad- 
dition, uniformity of amplification by the various zones of the 
clear aperture has been obtained in a higher degree than could 
hitherto be done. 
The objectives will be specially useful in photo-micrography 
where the correction of the secondary spectrum will be found of 
considerable practical advantage. Not only is there no differ- 
ence in the optical and chemical foci, but the image formed by 
the chemical rays is in itself much more perfect. This advan- 
tage is very clearly verified by experimental trials which have 
