Optical technology

The use o f optical instruments began in ancient Greece during warfare activities. The study o f Euclidian geometry had already uncovered the law o f light reflection, and enlistment o f mirrors as defence devices can be found in many historical battles. Later, the Greek-Egyptian philosopher Claudius Ptolemaeus (ca. 100-170), made a first scientific approach on the law o f refi*action. He found through an experiment that when a light ray travels inside water, it bends. He tabulated results o f refi*action for various incident angles. The phenomenon was not studied further until the beginning o f the 13*** century, when Robert Grosseteste (ca. 1168-1253) studied how a lens brings light to a focus: from rays refracting at each surface of the lens. The latest theory triggered the production o f spectacles.

2.2.1 Telescopes

The first clear lenses were manufactured in Italy, late in the 16* century. Optical experimentation using a combination o f convex and concave lenses led to the invention of the first telescope in September 1608 by Hans Lippershey (1587-1619), a Dutch spectacle maker. A foot-long tube, with a concave eyepiece at one end and a weaker convex lens at the other, could make distant things seem closer.

Galileo made the first telescopes for observing planetary motions. He managed to manufacture an objective lens with a magnification power o f 30x, which allowed him to observe the texture o f the moon’s surface. Soon, other scientists started making their own telescopes. However, all o f them suffered from three effects: low image brightness, spherical chromatic aberrations, and small magnification power. In search o f a solution, researchers designed two types o f telescopes: one that used mirrors (reflecting

telescope) to guide more light to the object, and another that combined a number o f

Pr e l im in a r ie s

former telescopes were suffering from spherical aberrations, the latter suffered from low magnification power. With the development o f silver-coated glass mirrors, the production o f reflecting telescopes surpassed that o f refracting ones. Nowadays, reflecting telescopes are equipped with a thin figured transparent plate mounted at the front end that acts as a refractor and allows the reflector to give a sharp image over the entire field o f view.

2.2.2 Microscopes

It is believed that the first microscope was created by Galileo in an attempt to study insects. Nevertheless, the invention o f the first compound"^ microscope was attributed to a Dutch spectacle maker, Zacharias Janssen (ca. 1595).

Originally, microscopes were used for the study o f insects and bacteria. New, improved instruments were manufactured, offering better image brightness, higher magnification and greater resolution. Anthony van Leeuwenhoek (1632-1723), a Dutch master o f the art, made a single lens microscope that had a magnification power o f 25Ox. His instruments were very simple, made out o f a single lens from glass or quartz crystals. The lens itself was very small and curved, very finely ground and polished. Unfortunately, the small size o f the lens made long viewing tiresome and uncomfortable. Moreover, like telescopes, the quality o f the new instrument was limited due to the physical properties o f the single lens.

The first drawings o f the microscopic world were drafted by Robert Hooke (1635- 1703), and presented in his book Micrographia (see Figure 2.1). His microscope was based on the Galilean model and consisted o f two lenses: a convex front lens formed a magnified image o f the object and a second concave, called the eyepiece, magnified the image further. The microscope is illustrated in Figure 2.2.

Pr e l i m i n a r i e s

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Figure 2.1: Magnified image of mould growing on the leaf of a rose (Micrographia, 1667).

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Figure 2.2: Hooke’s microscope.

The instrument began to take its traditional shape at the beginning of the 19^^^ century with the contribution of Carl Zeiss (1816-1888). The German engineer opened the first microscope production workshop in 1846 with the aim of producing high-precision microscope for dissections. Soon, he realised that no matter how precisely his microscopes were made; the only way to make them better would be to establish a computational rather than experimental design for his instruments. He went on to collaborate with Ernst Abbe (1840-1905), a mathematician and physicist at the university of Jena. A few years later, Abbe developed his wave theory for microscopic imaging and for the first time microscopes were assembled according to theoretical considerations. In 1886, Otto Schott (1851-1935), a glass chemist and new member of the Zeiss-Abbe partnership, produced a range of new objective lenses that were free from chromatic aberrations^.

T his type o f lens is called apochrom at as it produces images with no colour distortions.

Pr e l i m i n a r i e s

The full potential of Abbe’s mathematical designs was not reached until August Koehler (1866-1948) devised a new illumination method for microphotography in 1893. Three years later, Horatio S. Greenough designed a light microscope, consisting of two objectives and two oculars, which allowed the perception of depth in a microscopic image (see Figure 2.3). An alternative microscope design for depth effects, consisting of a single objective lens and two oculars (CMO), was introduced early in the 20^ century. Both the Greenough and the CMO microscope designs are explained further in Chapter 3.

Figure 2.3: Early 19^ century, G reenough-type, microscope.

Abbe’s designs and Koehler’s illumination method form the basis of light microscope development up to this date. Still, the theory of optical instruments has proved that there is a limit to the power of light microscopes. Optical studies have shown that no light microscope can clearly visualise any detail smaller than 200 nm. However, the applications of light microscopy have become almost endless, with microscopes commonly found in all kind of laboratories, industrial sites and hospitals.