Sagot :
1. Swelling of tissue components
2. Shrinkage of tissue components
Artifact types 1 and 2 are the result of poor fixation and/or dehydration techniques, i.e. osmolarity of the fixative may be wrong, pH may be wrong, too short a fixation time was used, and/or dehydration of the tissue was too rapid. Swelling and shrinkage can sometimes result in rupture of membranes. This sort of damage is particularly evident at the ultrastructural level.
3. wrinkles in section
4. tears in section
5. air bubbles
6. dust
Artifact types 3, 4, 5 and 6 are usually the result of poor sectioning technique or poor technique during mounting of sections. In some cases, poor fixation and/or embedding can be responsible for tears or wrinkles in sections by modifying fixed tissue consistency such that the tissue cannot be sectioned without its tearing or wrinkling.
7. stain precipitate
This sort of artifact can result from use of old stain solutions, use of unfiltered stain solutions, mistakes made during preparation of the stain, or poor staining technique.
THE LIGHT MICROSCOPE:
We have gone over the use of your light microscopes during lab and your laboratory handout has instructions that describe how to set-up your microscope for viewing such that "proper Kohler illumination" is established. In setting up "proper Kohler illumination" you are adjusting the microscope illuminatin such that 1) all light passes through the centers of the lenses and 2) the light beam is set at its smallest useful diameter thus eliminating reflections of light off of internal components of the microscope.
The end result of your adjustments for "proper Kohler illumination" is that you are able to view tissue sections at the highest possible resolution that your microscope is capable of. This means that you will be able to see the maximum amount of structure within the tissue that can be seen with your microscopes.
The objective and ocular lenses are responsible for magnifying the image of the specimen being viewed.
Total magnification = Objective magnification X ocular magnification
So for 10X objective and 10X ocular,
Total magnification = 10 X 10 = 100X (this means that the image being viewed will appear to be 100 times its actual size).
For a 40X objective and 10X ocular,
Total magnification = 10 X 40 = 400X
Magnification is not of much value unless resolving power is high.
Resolution is a measure of the ability to distinguish 2 points as two points. That is, when viewing something through a microscope, how close together can two points
be placed such that you can still see some space between them?
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We can't say much more about resolution without a few words about numerical aperture (n.a. or NA). The value for numerical aperture measures to what extent the light that passes through a specimen is spread out over and collected by the objective lens. The light that passes through the specimen contains information about what the specimen looks like, that is, about its structure.
If we consider the cone of light that originates from the specimen and enters the objective lens, Numerical aperture can be defined as,
NA = n . sin m ( . is the multiplication symbol)
n = refractive index of substance between the specimen and the objective lens (usually air, n = 1.0; quartz, n = 1.5; glass, n= about 1.5; water, n = 1.3)
m = 1/2 the aperture angle (also called the semiangle). The aperture angle is the angle described by the cone of light that enters the objective lens after passing through the specimen. This angle will depend on the curvature of the lens and also on how close the objective lens is to the specimen when it is in focus.
So, for an objective with an aperture angle of 120 o with air between specimen and objective lens,
NA = 1 . sin 60o = sin 60 o = 0.87
If oil with refractive index of 1.5 is used between the objective lens and the specimen,
NA = 1.5 . sin 60 o = 1.5 (.87) = 1.31
Now, numerical aperture is important because it allows us to calculate the resolving power of the objective. Remember, that's what we really were interested in determining initially.
R = 0.61 . ( l / NA)
R = resolution of the objective
l = wavelength of light (average value for white light ~ 550 nm).
NA = numerical aperture
So, for air situation,
R = 0.61 . 550nm/.87 = 386 nm = 0.000000386 m = 0.386 mm
For oil immersion,
R = 0.61 . 550nm/1.31 = 256 nm = 0.000000256 m = 0.26 mm
Thus, one can see that higher resolution is possible if the substance lying between the specimen and the objective lens has a refractive index as close as possible to that of the lens itself without exceeding the lens' refractive index.
It is important to realize that while both the ocular and objective lenses are responsible for the final magnification on a compound microscope, ONLY the objective lens is responsible for resolution.