In a historical and simplified way, we can classify microscopes into: lenses or hand magnifiers, simple microscopes and compound microscopes.

Glasses or handheld magnifying glasses. They are known as pocket magnifiers. The magnifying lens is mounted on a metal or plastic ring and achieves magnifications of 6 to 10 diameters. The figure shows the combination of glasses used to correct aberrations and obtain an image without distortion or color (chromatic aberrations). They can have a diaphragm to regulate the light, thus obtaining high quality images of up to 20x. In the collection are the following 26, 26, 29, 31 and 34.

Simple microscope. It consists of a magnifying glass mounted on a mechanical system that allows a smooth focus movement. The object or sample to be observed is located on a support or stage that has a central hole to allow the passage of light rays that are reflected by the mirror located at the base of the apparatus. The magnification is achieved by a single lens system. So are the first microscopes in the collection No. 25 to 27.

Compound microscope is one that produces an enlarged image by means of two optical systems (each formed by one or more lenses) that act successively. It is distinguished from a simple microscope or magnifying glass in that it magnifies the object by means of a single optical system. (usually a single lens). Compound microscopes are used to obtain large magnifications, between 40 and 1,500 times of a transparent object, which is illuminated from the other side, against the light (transparent illumination).

The simple microscope is one that uses a single lens to magnify the images of the objects observed. It is the most basic microscope whose classic example is the magnifying glass.

More than five hundred years ago, simple glass loupes were developed. These were convex lenses (thicker in the center than in the periphery). The sample or object could then be focused by the use of the magnifying glass placed between the object and the eye. These «simple microscopes» could diffuse the image onto the retina by magnification by increasing the visual angle on the retina.

Microscopio simple. Diagrama óptico de un microscopio simple

Diagrama óptico de un microscopio simple.

Microscopio simple de Leeuwenhoek

Microscopio simple de Leeuwenhoek

Left. Optical diagram of a simple microscope. It is observed that the virtual image created by the lens is greater than the object of study. Right. Simple Leeuwenhoek microscope. The sample was placed on the tip of the screw, in front of the single lens, embedded in the hole, of which the smallness of its diameter stands out.

The object to be observed is placed between the focus and the surface of the lens, which determines the formation of a virtual image, right and greater the greater the diopter power of the lens and the further away the near point is from the clear vision of the subject. The magnification obtained with these microscopes is small, because the wavelength of visible light imposes limitations on it.

The Dutchman Anton van Leeuwenhoek is credited with introducing biologists to the use of the microscope, despite the fact that simple magnifying glasses were already in production in the 16th century. Leeuwenhoek built very efficient microscopes based on a single lens. His observations were famous enough to communicate regularly with the Royal Society of London and receive numerous royal visitors.

These simple single-lens microscopes produced a magnification of up to 275 times (275x) and had a resolving power of 1.4 µm .; they did not suffer from the aberrations that limited the effectiveness of early compound microscopes, such as those used by Robert Hooke. With them Leeuwenhoek was even able to describe for the first time microscopic free-living and parasitic protists, sperm cells, blood cells, microscopic nematodes, rotifers and even bacteria.