AGf~ICULTURAL RESEARCH, PUSA.

·4,8 PRAC1'ICAL BACTERIOLOGY
Makers now engrave the initial magnification of
the objective on the objective mount, and as the eyepieces
are also designated by their magnifying power,
the total magnification is easily and correctly determined,
provided 'of course that the proper tube
length is used. '
'rhe h;-in. objective has, in reality, a shorter focal length
than that by which it is designated, and gives a magnification
of 100 diameters. ·When used in conjunction with a 10 x
eye-picce the total magnification is 1000 diameters.
The magnification u8ually employed_in bacteriological work·
is 800-1000 diametcrs.
The ,\-in. oil-immersion lens works very close to the
cover-slip, and the intervening space between objective
and cover-slip is filled with cedar-wood inunersion oil.
The reason for this is that when an oblique ray of light
emerges from a dense medium (glass) into a rare medium
(aiI-) it is refmcted outwards-i.e. away from the
normal (see diagram, p. 47-ABCD). As the brightness
of the image depends upon the light entering the objective,
and the resolution (viele infra) depends on the
effective aperture, this refraction of light diminishes
not only the brightness but the clearness of the image.
If, however, the space betwcen objective and object
is occupied by immersion oil, which has the same
refractive index as glass, the rays of light do Hot
undergo refraction and pass into the objective (sec
diagram-FBEG). .
The high-power (iJ-in.) is a " dry" lens and must nut be llsed
with immersion oil. Oil must be used only with lenses specially
computed to work with this fluid. Such objectives lIavC "oil
immersion" engraved on them.
NUMERICAL APERTURE
Objectives are rated not only by their focal length
but also by their Numerical Aperture (N.A.). The
numerical a.perture may be defined simply as the