Bright Field Microscope
A Bright Field microscope is the most basic type of microscope almost everyone has seen or had the opportunity to play with sometime in their lives. It consists of a plate over which is suspended a tube. This tube contains an eyepiece connected to an interior lens called an ocular. The ocular is then connected to an objective lens, which is the lens assembly that focuses and magnifies light passing through it. Typically there are several lenses on a circular dial which can be switched to see an item at specific levels of magnification. A specimen is placed on a perfectly transparent glass rectangle. It is clipped into place on the center of the plate. Below the glass is a hole in the plate, beneath which is a light source. Cheap microscopes have mirrors, but professional Bright Field microscopes include a light bulb. The light bulb is turned on, the light passes through the glass above it, through the lens, through the ocular, into the eye of the observer. The image can be focused by means of two dials on the sides of the microscope, which move the tube back and forth until the lens in the ideal distance from the specimen for the clearest picture possible.
Dark Field Microscope
The Dark Field microscope is similar in form and function to a Bright Field microscope. The main advantage to this type of microscope is that it enhances contrast between the specimen and surrounding bodies. Inability to see where one organism or structure stops and the other begins is the biggest problem with a Bright Field microscope. Light is emitted beneath the sample as usual, but it does not have a straight line to the sample plate. Instead there is a small disc in the way which occludes some of the light, only letting light pass around the disc. This light passes through a condenser lens and is focused on the sample plate, which it subsequently passes through. Some light, upon direct contact with a solid object within the sample, is scattered outward. That light is picked up while the light which passed directly through the sample plate is occluded by another disc called a direct illumination block. The scattered light is passed through the lens, is focused, and passed on up to the ocular for viewing.
A Fluorescence microscope is a device used specifically for examining organic specimens only. This is because it relies on the intended specimen to react to a specific wavelength of fluorescent light which is typically beyond the capacity for the human eye to see. The light is passed through a focusing lens, then through the specimen. The specimen reacts by bending the light, creating a slightly longer wavelength of light to be reflected than that which was originally used. A photosensitive lens is placed over the magnifying lens. This lens is set to respond to a wavelength within a range somewhat greater than that which was used. The lens reacts to this wavelength by making the invisible light visible to the human eye, making only the part of the specimen one is interested in easily studied.