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Unformatted text preview: f smaller structures where high resolution is more important. High power lenses
with higher numerical apertures are most useful here.
Parfocality of the objective lenses. A microscope is said to be parfocal if the object remains
in focus (or close to focus) as the nosepiece is rotated to bring different objective systems into position.
Are our microscopes parfocal? This is determined as follows:
a. Using the techniques described in section #II in the SUMMARY OF MICROSCOPE USE
PROCEDURES, place a slide with the letter "e" on the stage of your microscope and obtain a
sharply focused image of the "e". Center the “e” in the field of view.
b. Using the proper technique, rotate the nosepiece to bring the medium power objective into
alignment. Does the letter "e" remain in focus (or close to focus)? Before going to step c,
sharpen the focus and recenter the “e” in the field of view.
c. Repeat above step with the high power objective. Does the letter "e" remain in focus? (Do not
change objectives or adjust the focus at this time – there is another observation that you will need to
make with this slide!) If the answers to questions b & c are "yes", the microscope is parfocal.
Why is parfocality important to the microscopist? The answer is simple and twofold:
First, it makes specimens easier to find at high magnification. This is true because a parfocal
lens system allows one to find the specimen at low power, and then maintain this focus when a higher
power objective is selected. Second, it prevents damage to microscopes and slides. When prefocused
higher power objectives are rotated into position, there is little chance of crashing objective into slide.
Working Distance of the objectives lenses. As mentioned previously, working distance for a
focused objective is the distance between the tip of that objective and the specimen being viewed.
Working distance varies greatly between the objectives supplied with our microscopes. To observe
this variability, do the following.
a. Starting with the microscope configuration achieved in part “c” of the Parfocality exercise (40x
objective focused upon the letter “e”) observe the working distance of the 40x objective.
b. Rotate the 10x objective into position, focus, and note the working distance.
c. Finish by rotating the 4x objective into position, focus, and note the working distance.
Here you should see that working distance decreases with an increase in the power of the
objective. While an awareness of the working distance for the different objectives has no direct
influence upon the quality of the image, it does have two practical uses. First, the microscopist must
always be mindful of the short working distances of high power objectives and the potential for
unintended contact between the objective and the slide. This potential problem is averted by always
beginning observations on a new slide with lower power objectives. Second, the long working distance
of the low power (4x) objective offers plenty of room for exchanging slides and eliminates the need for
lowering the stage when a new slide is selected. Biology 05LA – Fall Quarter 2012 Lab 1 – page 6 4. The Ocular Lenses. The ocular lenses use the real image formed by the objective lens as its object
and produces the virtual image that the viewer sees (please see Figure. 3). The reason that the
virtual image has this name is because it has the “virtual” appearance of being present in a plane
located below the microscope. Once again, this type of microscope is called “compound” because
the image that the viewer sees is formed by two lenses in
upon retina Observations relevant to the virtual image of the
Obtain a focused image of the letter “e” at low power.
What direction does the image move if you move the
slide toward you? Away from you? To the right? To
the left? Is the image upright or inverted?
The presence of two ocular lenses (one for each eye)
provides an advantage to the microscopist as well as a
disadvantage. The advantage is that, with a binocular
viewing system, the microscopist has greater depth
perception of the selected field of view. The disadvantage
is that because of individual differences in vision, the
oculars need to be adjusted to accommodate different users.
These adjustments are made as follows:
Adjustment of inter-ocular distance (I.O.D.). While
looking through the microscope, the eyepieces are
moved apart or together to accommodate the varying
I.O.D.'s of different viewers. Lens and iris
of the eye Ocular Real image
formed by the
objective lens Objective lens Condenser
lens Orientation of
Illuminator lens Adjusting focus of the ocular lenses. Note that one of
the oculars has a fixed focus (right side) while the other
is adjustable with the diopter ring (left side). This
permits the viewer to compensate for differences in the
Figure 3 An
Figure 2 An image form...
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This note was uploaded on 08/27/2013 for the course BIO BIOL05LA taught by Professor Abbottl during the Fall '12 term at UC Riverside.
- Fall '12