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The Intricacies Of A Modern Compound Microscope
By Wendy Butler
The compound microscope is one form of microscope that uses a single glass lens of short focal length for the object while another single glass lens for the ocular or eyepiece. The modern compound microscope is generally far more complex in nature. These deal with multiple lenses. These lenses are on both the ends i.e. objective and eyepiece assemblies.
Chromatic aberration and spherical aberration appear in a much more reduced form when these multi-component lenses are used. To provide stable, controllable illumination the mirror is replaced by a lamp unit in a modern compound microscope.
It is basically an uncertain criterion while indicating the inventor of the compound microscope. It is often and generally believed that in 1590 two persons, father and son, spectacle-makers from the Netherlands, invented the first compound microscope. Their names were Hans Janssen and Zacharias Janssen. But there is a real heavyweight contender for the title. Galileo Galilei. In 1609 Galileo Galilei developed an 'occhiolino' or compound microscope. It comprised of a convex and a concave lens. In the late 1600s Christiaan Huygens, another inventor from the Netherlands also developed a two-lens or a compound microscope.
But whoever invented it, the compound microscope follows almost the same principal till date. The compound microscope could be divided into eight major parts depending on their functionality. They are ocular lens or eye-piece, objective turret or nosepiece, objective lenses, coarse adjustment knob, fine adjustment knob, object holder or stage, mirror and diaphragm and condenser. Each of the parts synchronizes with the other and this reveals an image that is magnified up to 1000Ã. A compound microscope is used on specimens that have a very limited depth of field i.e. very minute or thin specimens.
A typical compound microscope is composed of three main objective lenses. The first one is the scanning lens. This one has a magnifying power of 4Ã--. Then there is a low power lens with a magnifying power of 10Ã--, and a high power lens with a magnifying power of 40Ã--. In the more advanced version of the compound microscope there is an additional lease present. This fourth objective lens is an oil immersion lens. This oil immersion lens can champion a magnifying power of about 100Ã--.
According to the Encyclopedia, 'Optical microscopes are restricted in their ability to resolve features by a phenomenon called diffraction which, based on the numerical aperture (NA or /A/_/N/) of the optical system and the wavelengths of light used (λ), sets a definite limit (/d/) to the optical resolution. Assuming that optical aberrations are negligible, the resolution (/d/) is given by: d = frac {lambda} {A_N} usually, a λ of 550 nm is assumed, corresponding to green light. With air as medium, the highest practical /A/_/N/ is 0.95, and with oil, up to 1.5. Due to diffraction, even the best optical microscope is limited to a resolution of 0.2 micrometers.'
On the whole a compound microscope is the most widely used and vastly accepted microscope in the world.
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