2 
ELEMENTARY CHEMICAL MICROSCOPY 
whose principal focus lies at the distance marked upon the ob¬ 
jective. 
In a similarly constructed series, the smaller the value of the 
equivalent focus, the greater will be the magnifying power of the 
objective. A few manufacturers still arbitrarily letter or number 
their objectives. In such cases it is generally the rule that the 
earlier in the alphabet the letter or the smaller the number in 
the series the lower the magnifying power. 
When properly focused upon a preparation, the front or lowest 
lens entering into the construction of an objective is usually 
nearer to the preparation (in dry objectives) than the distance 
indicated by the equivalent focus. This distance between the 
front combination of the objective and the preparation, when in 
focus, is known as the working distance of an objective. In their 
selection for use in microchemical analysis the working distance 
becomes one of the most important considerations affecting the 
choice of the objectives. 
The construction of typical microscope objectives is shown 
diagrammatically in Figs. T6, 17, and 18. 
All objectives are corrected to a greater or lesser degree for 
chromatic aberration (presence of colored fringes around the 
images) and also largely for spherical aberration (failure to yield 
a flat field of view). When the spherical aberration is so corrected 
as to yield an especially large and flat field the objectives are 
often called aplanatic objectives. Although an objective may 
be so corrected as to yield a flat field, images of objects lying near 
the circumference are apt to be hazy or indistinct, the result of 
a form of spherical aberration known as coma; this is especially 
marked in high power objectives and requires unusual care in 
construction for its elimination.^ 
In all ordinary so-called achromatic objectives the corrections 
are usually such as to bring the rays of two spectral colors to a 
focus. In such lenses the optical and chemical foci may lie in 
different planes and therefore such objectives may not give 
really good results if employed in photomicrography; for this 
reason specially corrected achromatic lenses called photo- 
^ See Spitta, Microscopy, London, 1909, 
