36 PART II A Deeper Look into a Light Microscope
they provide better resolution and contrast. These objectives are also
called
fluorites, referring to the calcium fluorite, which is used for their
construction.
✔✔Apochromat: If you use apochromatic objectives, your images are free of
traces of color fringes. This is achieved by a clever combination of different
materials with opposite refraction behaviors. Apochromatic objectives
bring three wavelengths, typically red, green, and blue, into the focus on
the same plane.
Additionally, they are corrected spherically for either two
or three wavelengths. It’s no surprise that apochromatic objectives offer the
highest level of color correction.
Oil or Water? Increasing the Resolution
Already in 1847, the Italian physicist Giovanni Battista Amici demonstrated that
image sharpness and brightness increase dramatically upon direct contact between
the front lens of the objective and the sample by means of a liquid. In other
words, if you fill a media between the objective lens and the specimen, you can
increase the numerical aperture to achieve higher resolution. These media are
referred to as immersion liquids. Typical immersion liquids are synthetic immersion
oil, glycerole, water or silicone oil. The indication »Oil«, »Glyc«, »W«
or »LCI« on the objective itself tells you which immersion fluid you should use.
In contrary, dry objectives are designed for use without an immersion fluid. They
are typically of low to mid magnification and you can use them with or without
cover glass.
The Thickness of the Coverslip Glass
To visualize samples such as bacteria, cell cultures or blood, you normally use a
coverslip glass to protect your little fellows from contamination. But physics and
especially optics is a tricky thing. The use of a glass coverslip changes the way
light refracts from the object into the objective. As a result, the objective needs
to make proper optical corrections to produce the best quality image. This is the
reason why some objectives can be adapted to a range of coverslip thicknesses for
which they are optimized.
Planar Objectives Provide Flat Images
Objective lenses are curved – convex or concave, but either way curved. This design
results in field curvature, showing images that are accordingly also curved