Compound microscope

The compound light microscope has the ability to provide different magnifications by changing the "objective" lens. It also permits a greater working distance from a specimen by allowing for finite focal lengths outside of the lens itself.

In the case of the single lens, greater magnification is obtained by increasing the curvature of the surface of the lens. In theory, the greatest magnification would result from a perfectly spherical lens. However, the focal length of the lens would be on the surface of the lens, and the field of view would be infinitely small, making the device impractical. Using two lenses in tandem enables magnification to be viewed as the product of the combined magnifications of the individual lenses and allows for a significant working distance from the specimen with a relatively large field of view.

It was the physicist, Ernst Abbé working with the master lens maker, Carl Zeiss in Jena, Germany during the latter part of the 19^th century who defined the rules of light optics, and who determined the theoretical limits of resolution. To their amazement, it was found in the 1880's that Zeiss had already produced (through his own skills) microscopes that had essentially reached the limits of resolution (approximately 0.2 µm).