50 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
objectives the spherical aberration is usually undercorrected for the red 
and overcorrected for the blue rays ; they exhibit, in other words, chromatic 
undercorrection for the axial zone and overcorrection for the marginal zone. 
The equivalent focal length of the objective varies with the color, being 
shortest for the yellow-green and increasing for both ends of the spectrum, 
thus giving rise to the "secondary spectrum." Semi-apochromatic objectives 
are spherically corrected for two colors; the chromatic difference of spherical 
aberration is eliminated, so that there is practically even chromatic correc- 
tion for the whole aperture of the objective. Semi-apochromats do not 
differ, however, from achromats in respect to the secondary spectrum. In 
apochromutic objectives the secondary spectrum is absent. They are apla- 
natic for two colors (*. e., are spherically corrected and fulfill the sine condi- 
tion for two colors). The chromatic difference in magnification (the E. F. 
of the objective is shorter for blue rays than for red) which remains in the 
apochromat,as in all objectives with non-achromatic front lens, can be elimi- 
nated by the use of compensating eye-pieces which work well with high- 
aperture achromats and semi-apochromats, but to best advantage with 
apochromats where the chromatic difference of magnification is the same for 
all zones of the objective. Low-power achromats show practically no chro- 
matic difference of magnification and exhibit, in consequence, colors near the 
margin of the field when used with overcorrected compensating eye-pieces. 
In low-power apochromats, which might also be made free from chromatic 
difference of magnification, the amount of this aberration which is present 
in the high-power objectives is introduced purposely in order that the com- 
pensating eye-pieces can be used with them as well as with the high-power 
objectives. In the apochromatic objectives, which were first devised by 
Abbe, practically all the corrections are carried to greater refinement than 
in the ordinary achromatic objectives, especially for the central part of the 
field. The definition which is the result of the designer's close figuring and 
the operator's workmanship is consequently better in an apochromat than 
in any other objective. 
In very low power objectives, the object field is proportionately larger 
and the aberrations (astigmatism, etc.) due to oblique rays from points 
distant from the axis become more prominent and have to be taken into 
account by the lens designer. 
Objectives are also classified as dry and immersion object glasses respec- 
tively, according as air or a liquid is to serve as medium between the cover- 
glass and the front lens of the mount. If the liquid and front objective lens 
are of the same refractive index the immersion is said to be homogeneous. 
The immersion lenses, although not often used in petrographic microscopic 
work, are optically superior to the dry series of objectives because of their 
higher numerical aperture, better optical corrections, longer working dis- 
tance, and freedom from aberrations due to variability in thickness of 
cover-glass and freedom from loss of light by reflection at the surface of the 
cover-slip and the front lens of objective. Notwithstanding these advan- 
tages the oil-immersion lenses are rarely used in petrographic work, because 
of the relatively low magnifications there required.* 
It may be noted in passing that xylol or benzene and not alcohol should be used in cleaning the liquid 
from an immersion objective. 
