edium-power" lens--e. g., 1/6 inch or 1/8 inch (dry). These lenses
must be carefully selected, especial attention being paid to the
following points:

(a) _Correction of Spherical Aberration._--Spherical aberration gives
rise to an ill-defined image, due to the central and peripheral rays
focussing at different points.

(b) _Correction of Chromatic Aberration._--Chromatic aberration gives
rise to a coloured fringe around the edges of objects due to the fact
that the different-coloured rays of the spectrum possess varying
refrangibilities and that a simple lens acts toward them as a prism.

(c) _Flatness of Field._--The ideal visual field would be large and,
above all, _flat_; in other words, objects at the periphery of the field
would be as distinctly "in focus" as those in the centre. Unfortunately,
however, this is an optical impossibility and the field is always
spherical in shape. Some makers succeed in giving a larger central area
that is in focus at one time than others, and although this may
theoretically cause an infinitesimal sacrifice of other qualities, it
should always be sought for. Successive zones and the entire peripheral
ring should come into focus with the alteration of the fine adjustment.
This simultaneous sharpness of the entire circle is an indication of the
perfect centering of the whole of the lenses in the objective.

[Illustration: FIG. 48.--Huyghenian eyepiece.]

(d) _Good Definition._--Actual magnification is, within limits, of
course, of less value than clear definition and high resolving power,
for it is upon these properties we depend for our knowledge of the
detailed structure of the objects examined.

(e) _Numerical Aperture_ (_N. A._).--The numerical aperture may be
defined, in general terms, as the ratio of the _effective_ diameter of
the back lens of the objective to its equivalent focal length. The
determination of this point is a process requiring considerable
technical skill and mathematical ability, and is completely beyond the
powers of the average microscopist.[1]

Although with the increase in power it is correspondingly difficult to
combine all these corrections in one objective, they are brought to a
high pitch of excellence in the present-day "achromatic" objectives, and
so remove the necessity for the use of the higher priced and less
durable apochromatic lenses.

In selecting objectives the best "test" objects to employ are:

1. A thin (one cell layer), even }