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Light Microscopes
Ever since their invention in the late 1500s,light microscopes have enhanced our knowledge in basic biology, biomedical research, medical diagnostics and materials science. Light microscopes can magnify objects up to 1,000 times, revealing microscopic details. Light-microscopy technology has evolved far beyond the first microscopes of Robert Hooke and Antoni van Leeuwenhoek. Special techniques and optics have been developed to reveal the structures and biochemistry of living cells.Microscopes have even entered the digital age, using charge-coupled devices (CCDs) and digital cameras to capture images. Yet the basic principles of these advanced microscopes are a lot like those of the student microscope you may have used in your first biology class.

The Basics
A light microscope works very much like a refracting telescope, but with some minor differences. A telescope must gather large amounts of light from a dim, distant object; therefore, it needs a large objective lens to gather as much light as possible and bring it to a bright focus. Because the objective lens is large, it brings the image of the object to a focus at some distance away, which is why telescopes are much longer than microscopes. The eyepiece of the telescope then magnifies that image as it brings it to your eye.

In contrast to a telescope, a microscope must gather light from a tiny area of a thin, well-illuminated specimen that is close- by. So the microscope does not need a large objective lens. Instead, the objective lens of a microscope is small and spherical, which means that it has a much shorter focal length on either side. It brings the image of the object into focus at a short distance within the microscope's tube. The image is then magnified by a second lens, called an ocular lens or eyepiece, as it is brought to your eye. The other major difference between a telescope and a microscope is that a microscope has a light source and a condenser. The condenser is a lens system that focuses the light from the source onto a tiny, bright spot of the specimen, which is the same area that the objective lens examines. Also unlike a telescope, which has a fixed objective lens and interchangeable eyepieces, microscopes typically have interchangeable objective lenses and fixed eyepieces. By changing the objective lenses (going from relatively flat, low- magnification objectives to rounder, high-magnification objectives), a microscope can bring increasingly smaller areas into view -- light gathering is not the primary task of a microscope's objective lens, as it is a telescope's.

Making toy microscope
You can make a simple microscope by using magnifying glasses and paper: 1. Get two magnifying glasses and a sheet of printed paper. 2. Hold one magnifying glass a short distance above the paper. The image of the print will look a little bit larger. 3. Place the second magnifying glass between your eye and the first magnifying glass. 4. Move the second glass up or down until the print comes into sharp focus. You will notice that the print appears larger than it does in the first magnifying glass.

Image Quality
When you look at a specimen using a microscope, the quality of the image you see is assessed by the following:

· Brightness - How light or dark is the image? Brightness is related to the illumination system and can be changed by changing the voltage to the lamp (rheostat) and adjusting the condenser and diaphragm/pinhole apertures. Brightness is also related to the numerical aperture of the objective lens (the larger the numerical aperture, the brighter the image).

· Focus - Is the image blurry or well- defined? Focus is related to focal length and can be controlled with the focus knobs. The thickness of the cover glass on the specimen slide can also affect your ability to focus the image -- it can be too thick for the objective lens. The correct cover- glass thickness is written on the side of the objective lens.

· Resolution - How close can two points in the image be before they are no longer seen as two  separate points? Resolution is related to the numerical aperture of the objective lens (the higher the numerical aperture, the better the resolution) and the wavelength of light passing through the lens (the shorter the wavelength, the better the resolution).

· Contrast - What is the difference in lighting between adjacent areas of the specimen? Contrast is related to the illumination system and can be adjusted by changing the intensity of the light and the diaphragm/pinhole aperture. Also, chemical stains applied to the specimen can enhance contrast.