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Click Here For Best Selection Of High Quality Polarizing Microscope

Click Here For Best Selection Of High Quality Polarizing Microscope

The lower polar, condensing lenses, aperture diaphragm, auxiliary condensing lens, and one or more filter holders are the principal parts of the substage assembly. The assembly is most commonly mounted in a mechanism that would allow it to be raised or lowered anytime the observer wish.

 

            Most modern microscopes have lower polar. This lower polar is consists of a piece of optical-quality polarizing film mounted in a ring, which can be easily rotated at least about 90 degree and usually 180 degree or even 360 degree. This lower polar allows the vibration direction of the polarized light to pass through the microscope to be set in any desired orientation. The other term for polars is nicols. This is because nicol prisms were used to provide polarized light on many early petrographic polarizing microscopes. In many modern cases, this lower polar is usually mounted on a pivot so that is could be easy to swung it out of the optical path.

 

            An iris diaphragm or aperture diaphragm is usually mounted either above or below the fixed condensing lens of the microscope. The aperture diaphragm controls the size of the cone of light that pass through the petrographic polarizing microscope. When the aperture diaphragm is adjusted to nearly closing it, the size of the cone of light decreases and the contrast in the image seen through the microscope is increased. It is important to note that the function of the aperture diaphragm is actually not intended for the purpose of adjusting the illumination of the microscope. For this defined purpose, rheostat control or neutral gray filters should be used instead.

 

            In order to concentrate the light onto the area of the sample immediately beneath the objective lens, condensor lenses are used. It should be noted that a fixed condensor lens usually have numerical aperture, which about the same as the numerical aperture of the medium power objective lens. The illumination that is provided when the light reaching the sample from the fixed condensor is only moderately converging is called orthoscopic illumination.

 

            The other assembly that is mounted on a pivot so that it can be swing in and out of the optical path is called the auxiliary condensing lens. The main function of this assembly is to provide conoscopic illumination. This kind of illumination is consisting of strongly converging light. This assembly is usually swung into the optical path for the purpose of allowing the production of the optical phenomena called interference figures. These interference figures are examined with the high power objectives. However, not all microscopes are equipped with this auxiliary condensor. A fixed condensor is usually designed in order to provide a somewhat more strongly convergent light and also provide a compromise between the needs of orthoscopic and conoscopic illumination when this auxiliary condensor lens is not present. By adjusting the aperture diaphragm or by raising or lowering the condensor, some variation in the degree of convergence can be accomplished. However, this arrangement is not really very successful.

 

            Colored or gray filters on the other hand can be placed in the slots or in the swing out holders found at the bottom of most substage assemblies of petrographic polarizing microscopes. The intensity of the illumination is usually controlled by the gray filter. Colored filters are usually used to adjust the color balance of the light or to produce roughly monochromatic light.

 

Microscope Stage

 

            The petrographic polarizing microscope is usually found having circular stage. This stage is usually mounted on bearings so that it can be rotated smoothly. On the outside edge of the stage, there is a stage goniometer that is marked in degrees so that angles of rotation can be measured accurately. There are some times when a vernier accuracy is provided by the index mark. But then vernier accuracy is rarely needed. There are some microscopes designed of having a thumb screw or lever screw that can be engaged to lock the stage and also prevent it from rotating.



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Wednesday, February 27th, 2008 at 3:43 am
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Polarizing Microscope Resources
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