Compound Light Microscopes
The modern compound light microscopes are the collective product of over 400 years of attempting to view our microscopic world
. The compound microscope is special because of the lens used to magnify and view. Most optical microscopes used today are at their heart compound. This simply means that the microscope uses light and two lenses to view and contrast a specimen. These two lenses are the eyepiece, or ocular, and the objective lens.
The idea of using lenses to magnify and view objects was first addressed by Galileo, then used for microscopy by botanist Robert Hooke and Antonie van Leeuwenhoek was a tradesman and lens grinder. Hooke used a device to view cork cells and van Leeuwenhoek used possibly the first compound microscope to view animal cells. Today's compound light microscopes use cutting edge technology to push both magnification and resolution further than ever before. the first microscope's could magnify up to 266x, but with very low resolution. Magnification with some compound microscopes can reach 2000x, but the theoretical resolution limit in the visible light spectrum is only 1500x with clarity. Other light wavelengths must be used for higher magnification.
The first magnifying lens within compound light microscopes is the ocular. This lens is cut so that it provides the best view for the human eye. The lens is cut differently when a photolens is used for storage of images. The optical lens magnifies anywhere from 5x to 25x, with the typical ocular being 10x. The quality of this lens will greatly affect overall clarity.
The objective lens is the lens closest to the specimen. A range of magnifications are possible through rotating the objective lens. Being made parfocal, the same object or area remains in focus when it is rotated. Objective lens are often 10x, 40x, and the oil-, or water-immersion 100x objective. The oil or water at these higher magnifications disperses the view so that the specimen can be viewed more clearly.
Using these lenses in tandem allow for overall magnification, simply by multiplying the two magnifications. For example, a 10x ocular with a 40x objective yield a magnification of 400x.
The light source of compound microscopes is typically an incandescent bulb mounted below the stage on which the specimen rests. The light is projected upward through the stage and into the lenses to bring the contrast in the field of view.
Adjustments on the compound light microscope allow for more clear viewing, based on the vision of the viewer, the quality of the specimen, and the magnification desired. The optimum ocular adjustment is a set value for each viewer. Adjust the stage away from or to the objective to get the clearer view. The amount of light allowed to reach the lens can be adjusted by the iris diaphragm below the stage to view things clearer at different light levels, or to view living microorganisms that do not like bright light.
Using compound light microscopes can be fun, entertaining, and very beneficial. The advances in science from the use of this incredible device have allowed for a much broader and clearer view of our world, have helped to curb or eradicate disease outbreaks, have helped us understand genetic variations, and have been the needed evidence to solve many crimes.
by: Jacque Crook
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