Page 117 - The Veterinary Laboratory and Field Manual 3rd Edition

86 Willy Schauwers

Figure 2.49 Systematic examination of the microscopic field. Photo: Idzi Potters, ITG, Antwerp, Belgium.

For searching some of the protozoans the 40× index, it will be bent, away from the normal NN’
objective is used. In the same way as described of the optical system (left half of the scheme).
above, a few overlapping rows (3 or 4) should High-magnification objectives (50×, 100×)
be examined. A partial systematic examination need large amounts of light. To avoid loss of light
is used for permanently stained smears as well. by refraction, immersion oil with a sufficiently
Also here, the examined part should consist of high refractional index is placed between the
overlapping rows. preparation and the objective.
Some examples of different refractional indi-
ces:
Immersion objectives

Rays of light that pass the optical system of Air: n = 1.000
the microscope pass through different kinds of Water: n = 1.330
materials (glass, air). Each one of these materials Normal glass: n = 1.500
has a certain refracting capacity or refractional Immersion oil: n = 1.515
index n. When light passes through these mate- Canada balm: n = ± 1.5.
rials (with different refracting indices), the rays
of light will be bent and sometimes lost (when Extra topics available on the website:
they are bent so strongly they leave the optical
system of the microscope). The level and way • calibration of the microscope
of bending depend on the refracting capac- • Köhler illumination
ity of the environment and on the order of the • dark field illumination
transition. • phase contrast microscopy
When light passes from an environment with • fluorescent microscopes.
low refractional index to an environment with
higher refractional index, it will be bent to the
normal NN’ of the optical system (right half of
the scheme) (Figure 2.50). When light passes
from an environment with high refractional
index to an environment with lower refractional

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