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Microscopes are instruments designed to produce magnified visual or photographic images of small objects. The microscope must accomplish three tasks: produce a magnified image of the specimen, separate the details in the image, and render the details visible to the human eye or camera. This group of instruments includes not only multiple-lens designs with objectives and condensers, but also very simple single lens devices that are often hand-held, such as a magnifying glass.
The microscope illustrated in Figure 1 is a simple compound microscope invented by British microscopist Robert Hooke sometime in the 1660s. This beautifully crafted microscope has an objective lens near the specimen and is focused by turning the body of the microscope to move the objective closer to or farther from the specimen. An eyepiece lens is inserted at the top of the microscope and, in many cases, there is an internal "field lens" within the barrel to increase the size of the viewfield. The microscope in Figure 1 is illuminated through the oil lamp and water-filled spherical reservoir, also illustrated in Figure 1. Light from the lamp is diffused when it passes through the reservoir and is then focused onto the specimen with a lens attached to the reservoir. This early microscope suffered from chromatic (and spherical) aberration, and all images viewed in white light contained "halos" that were either blue or red in color.
Since so many microscope users rely upon direct observation, it is important to understand the relationship between the microscope and the eye. Our eyes are capable of distinguishing color in the visible portion of the spectrum: from violet to blue to green to yellow to orange to red; the eye cannot perceive ultraviolet or infrared rays. The eye also is able to sense differences in brightness or intensity ranging from black to white and all the gray shades in between. Thus, for an image to be seen by the eye, the image must be presented to the eye in colors of the visible spectrum and/or varying degrees of light intensity. The eye receptors of the retina used for sensing color are the cone cells; the cells for distinguishing levels of intensity, not in color, are the rod cells. These cells are located on the retina at the back of the inside of the eye. The front of the eye (see Figure 2), including the iris, the curved cornea, and the lens are respectively the mechanisms for admitting light and focusing it on the retina.
For an image to be seen clearly, it must spread on the retina at a sufficient visual angle. Unless the light falls on non-adjacent rows of retinal cells (a function of magnification and the spreading of the image), we are unable to distinguish closely-lying details as being separate (resolved). Further, there must be sufficient contrast between adjacent details and/or the background to render the magnified, resolved image visible.
Because of the limited ability of the eye's lens to change its shape, objects brought very close to the eye cannot have their images brought to focus on the retina. The accepted conventional viewing distance is 10 inches or 25 centimeters.
More than five hundred years ago, simple glass magnifiers were developed. These were convex lenses (thicker in the center than the periphery). The specimen or object could then be focused by use of the magnifier placed between the object and the eye. These "simple microscopes" could spread the image on the retina by magnification through increasing the visual angle on the retina.
The "simple microscope" or magnifying glass reached its highest state of perfection, in the 1600's, in the work of Anton von Leeuwenhoek who was able to see single-celled animals (which he called "animalcules") and even some larger bacteria with a simple microscope similar to the one illustrated in Figure 3. The image produced by such a magnifier, held close to the observer's eye, appears as if it were on the same side of the lens as the object itself. Such an image, seen as if it were ten inches from the eye, is known as a virtual image and cannot be captured on film.