Anatomy of a Microscope

What Is a Microscope?

Microscopes are instruments designed to produce magnified visual or photographic images of small objects. A microscope must accomplish three tasks: produce a magnified image, separate the details in the image, and render these details visible to the human eye or camera. This group of instruments includes multiple-lens microscope designs with objectives and condensers, as well as simple single-lens devices that are often handheld, such as a magnifying glass.

Parts of a Simple Compound Microscope

The microscope illustrated in Figure 1 below is a simple compound microscope invented by British microscopist Robert Hooke in the 1660s.

Parts of a Hooke Microscope

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 field of view.

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.

The Relationship Between Microscopes and the Eye

Since many microscope users rely on direct observation, it is important to understand the relationship between the microscope and the eye. Our eyes can distinguish color in the visible portion of the spectrum: from violet to blue to green to yellow to orange to red; however, the eye cannot perceive ultraviolet or infrared rays.

The eye can also 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 in colors of the visible spectrum and/or varying degrees of light intensity.

The receptors our eyes use for sensing color are called cone cells. The cells for distinguishing levels of intensity, not color, are the rod cells. Each of these cell types are located on the retina at the back of the inside of the eye. The front of the eye (see Figure 2), which includes the iris, the curved cornea, and the lens, admits light and focuses 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 nonadjacent 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). Furthermore, there must be sufficient contrast between adjacent details and/or the background to render the magnified, resolved image visible.

Because the eye's lens is limited in its ability to change shape, objects brought very close to the eye cannot have their images brought into focus on the retina. The accepted conventional viewing distance is 10 inches or 25 centimeters.

History of the Microscope

The Simple Microscope

More than 500 years ago, simple glass magnifiers were developed in the form of convex lenses (thicker in the center than the periphery). The specimen or object could then be focused by using 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, was optimized in the 1600s through the work of Anton von Leeuwenhoek. He 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 below.

The image produced by this magnifier, held close to the observer's eye, appears as if it were on the same side of the lens as the object. This type of image, seen as if it were 10 inches from the eye, is known as a virtual image and cannot be captured on film.

Parts of a Simple Microscope

The Compound Microscope

Around the beginning of the 1600s, through work attributed to the Janssen brothers in the Netherlands and Galileo in Italy, the compound microscope was developed (see the microscope in Figure 4).

Parts of a Compound Microscope

In its simplest form, the compound microscope consisted of two convex lenses aligned in series: an object glass (objective) closer to the object or specimen, and an eyepiece (ocular) closer to the observer's eye (with means of adjusting the position of the specimen and the microscope lenses). The objective projects a magnified image into the body tube of the microscope, and then the eyepiece further magnifies the image projected by the objective. Thus, the compound microscope achieves a two-stage magnification.

Compound microscopes developed during the 17th and 18th centuries were hampered by optical aberration (both chromatic and spherical), a flaw that is worsened by the use of multiple lenses. These microscopes were actually inferior to single lens microscopes of the period because of these artifacts. The images they produced were often blurred and had the colorful halos associated with chromatic aberrations that not only degraded image quality but also hampered resolution.

In the mid-1700s, lens makers discovered that by combining two lenses made of glass with different color dispersions, much of the chromatic aberration could be reduced or eliminated. This discovery was first used in telescopes, which have much larger lenses than microscopes. It was not until the start of the 1800s that chromatically corrected lenses became common in compound microscopes.

Advancements in Compound Microscopes

The eighteenth and nineteenth centuries saw a great improvement in the mechanical and optical quality of compound microscopes. Advances in machine tools enabled more sophisticated parts to be fabricated. By the mid-1800s, brass was the alloy of choice to produce high-quality microscopes.

Many British and German microscope manufacturers flourished during this time. Their microscopes varied widely in design and production quality, but the overall principles defining their optical properties remained relatively consistent. The microscope illustrated in Figure 5 below was manufactured by Hugh Powell and Peter Lealand around 1850. The tripod base provided a sturdy support for the microscope, which many people consider the most advanced of its period.

Parts of a Powell and Leland Microscope Diagram

The end of the 19th century saw a high degree of competition among microscope manufacturers. As a result, the development and production costs of microscopes became important. Brass, the material of choice for microscope manufacturers, is very expensive. It also was a lengthy task to machine, polish, and lacquer microscope bodies and other parts machined from brass. To cut expenses, microscope manufacturers first started to paint the exterior of the microscope body and stand, as well as the stage and other non-moving parts.

20th Century Microscopes

During the first quarter of the 20th century, many microscope manufacturers began substituting cast iron for brass in microscope frames and stages. Iron was much cheaper and could not be distinguished from brass when painted black. They also started to electroplate many of the critical brass components such as knobs, objective barrels, nosepieces, eyepieces, and mechanical stage assemblies (illustrated in Figure 6 below).

These early 20th century microscopes still subscribed to a common design motif. They were monocular with a substage mirror that was used with an external lamp to illuminate the specimen. A typical microscope of the period is the Zeiss Laboratory microscope pictured in Figure 6. This type of microscope is very functional, and many are still in use today.

Parts of a Zeiss Laboratory Microscope

Modern Microscopes

Modern microscopes far exceed the design specifications of those made prior to the mid-1900s. Glass formulations have vastly improved, enabling greater correction for optical aberration than ever before. Synthetic anti-glare lens coatings are now very advanced. Integrated circuit technology has enabled manufacturers to produce "smart" microscopes that incorporate microprocessors into the microscope stand. Photomicrography is easier than ever before with attachments that monitor light intensity, calculate exposure based on film speed, and automatically perform complicated tasks such as bracketing, multiple exposure, and time-lapse photography.

The microscope illustrated in Figure 7 is an Olympus Provis AX70 research microscope. Launched in the 1990s, this microscope featured a sophisticated design that incorporated multiple illuminators (episcopic and diascopic), analyzers and polarizers, DIC prisms, fluorescence attachments, and phase contrast capabilities. The photomicrography system featured spot measurement, automatic exposure control, and zoom magnification for flexible, easy framing. The Y-shaped frame helped improve ergonomics and ease of use. Today, microscope manufacturers continue to develop new microscope technology to improve user comfort, ease of use, and support new research.

Parts of an Olympus Provis AX 70 Microscope

Practical Uses for Microscopy

Practically everyone has, at one time or another, viewed the world through an optical microscope. For most people, this experience occurs during biology class in high school or college, although some scientific entrepreneurs have purchased their own microscopes either individually or as part of a science kit.

Photography through the microscope, or photomicrography, has long been a useful tool to scientists. The biological and medical sciences have relied heavily on microscopy to solve problems relating to the morphological features of specimens as well as a quantitative tool for recording optical features and data. In this way, the optical microscope has proven to be a useful tool for investigating the mysteries of life.

Microscopy has become a popular tool in the physical and materials sciences, as well as the semiconductor industry, due to the need to observe surface features of new high-tech materials and integrated circuits. Microscopy has also proven useful for forensic scientists who must examine hairs, fibers, clothing, blood stains, bullets, and other items associated with crimes. Modern advances in fluorochrome stains and monoclonal antibody techniques have paved the way for major growth in the use of fluorescence microscopy in both biomedical analysis and cell biology.

The Difference Between Biomedical and Materials Microscopy

The basic differences between biomedical and materials microscopy involves how the microscope projects light onto the sample. In classical biological microscopy, light is passed or transmitted through very thin specimens, focused with the objective, and then passed into the microscope eyepieces.

To observe the surface of integrated circuits (that comprise the internal workings of modern computers), light is passed through the objective and is then reflected from the surface of the sample back into the microscope objective. In scientific terminology, transmitted and reflected light microscopy are known as diascopic and episcopic illuminated microscopy, respectively. The photomicrographs in our photo galleries are derived from both transmitted and reflected optical microscopic scientific investigations.

A common issue in microscopy is the poor contrast produced when light passes through very thin specimens or reflects from surfaces with a high degree of reflectivity. To overcome poor contrast, various optical techniques have been developed to increase contrast and provide color variations in specimens. These optical techniques include:

• Polarized light
• Phase contrast imaging
• Differential interference contrast
• Fluorescence illumination
• Darkfield illumination
• Rheinberg illumination
• Hoffman modulation contrast
• Using various gelatin optical filters

Find a thorough discussion of these optical techniques in the specialized microscopy techniques section of this primer. For convenience, references are provided in both classical bibliographic form and as website links. These resources can help you learn about and educate others in microscopy and photomicrography.