Kōhler, August Karl Johann Valentin

(b. Darmstadt, Germany, 4 March 1866; d. Jena, Germany, 12 March 1948)

microscopy.

Köhler was the son of Julius Köhler, accountant to the grand duke of Hesse. After attending the Gymnasium and the Technische Hochschule in Darmstadt, he studied at the universities of Heidelberg and Giessen; when he took the state examination for teachers in 1888, had studied zoology, botany, mineralogy, physics, and chemistry. He taught in Gymnasiums in Darmstadt and Bingen until 1891, when he was appointed assistant at the Institute of Comparative Anatomy of the University of Giessen.

Köhler’s new design in 1893 for microscope illumination, which was to replace the existing condenser system, attracted the attention of the firm of Carl Zeiss in Jena; and six years later, Siegfried Czapski came to Bingen and invited Köhler to work in Jena for six months. In 1900 Köhler joined the Zeiss firm. He spent the rest of his life in Jena. In 1922 the university made him professor of microphotometry and projection, a post he held until 1945. He received honorary degrees from Edinburgh and Jena in 1934.

Köhler’s boyhood passion had been for geology, but his contact with the life sciences drew him to zoology; his earliest papers were on freshwater and land mollusks and his doctoral thesis (1893) on Siphonaria. The paper which brought him fame was “Ein neues Beleuchtungsverfahren für mikrophotographische Zwecke” (1893). From the start, Köhler’s aim had been to raise the standard and ease of microphotography. He began in 1893 by introducing the “collector” lens, which focused a magnified image of the light source in the plane of the condenser iris. By movement of the iris diaphragm, the size of the cone of illumination could be altered at will. This became known as the Köhler principle of illumination. Much later, at the Zeiss works, Köhler overcame the imperfections in microphotographs caused by the curvature of the image. The resulting fuzzy margins were avoided in Köhler’s negative “Homal” system (1922).

As a Gymnasium teacher in Bingen, Köhler had wanted to improve the resolving power (R) of the light microscope. Abbe and Zeiss had raised R by increasing the numerical aperture (NA) and in 1886 had introduced their apochromatic lenses, which pushed the optical microscope to the apparent limit of its resolving power. But Köhler wanted to go yet further by reducing the wavelength (λ) of the light source. From Abbe’s theory, the relation R =0.61λ/NA follows, and it can be seen that conversion from visual light of λ = 5500Å to ultraviolet of λ = 2750Å should yield a twofold increase in resolving power.

In the summer of 1900 Köhler, Moritz von Rohr, and Hans Boegehold began to work on the ultraviolet microscope. They began by attempting to make an objective suitable for light of short wavelengths. By 1902 they succeeded, but only for the green mercury line. A further two years passed before Köhler succeeded in designing a lens suited to the ultraviolet spectrum of cadmium. This objective, known as the “monochromator,” was used in the ultraviolet microscope which he described in 1904. Although Köhler, von Rohr, and their colleagues had overcome numerous difficulties, from the design of the cadmium arc light to that of the fused quartz lenses, the instrument was still difficult to use. Direct focusing and viewing were, of course, impossible.

Although their microscope was shown to the medical profession in Vienna in 1905 and offered for trial, there was little enthusiasm. Köhler’s fine pictures of the chromosomes in the epithelial cells of salamander gill buds, in which he noted the strongly ultraviolet absorbing character of the chromatin, were forgotten; and it was not until Tobjörn Caspersson made a thorough study of the absorption spectra of cell constituents some thirty years later that the ultraviolet microscope became popular. Since that time, the establishment of phase and fluorescent microscopy and closed circuit television conversion of the ultraviolet image have transformed the instrument which Köhler designed into a useful tool for the student of the living, unfixed cell.

BIBLIOGRAPHY

I. Original Works. Seventy-two papers by Köhler are listed in Reinert’s obituary notice (see below). The majority of his papers appeared in the Zeitschrift für wissenschaftliche Mikroskopie und für mikroscopische Technik, and of these the most important are “Ein neues Beleuchtungsver-fahren für mikrophotographische Zwecke,” 10 (1893), 433- 440; “Beleuchtungsapparat für gleichmässige Beleuchtung mikroskopischer Objekte mit beliebigem einfarbigem Licht,” 16 (1899), 1-28; “Mikrophotographische Untersuchungen mit ultraviolettem Licht,” 21 (1904) 129-165, 273-304.

Reports on his later studies, found in Naturwissenschaften, are “Einige Neuerungen auf dem Gebiet der Mikrophotographie mit ultraviolettem Licht,” 21 (1933), 165-172; and “Das Phasenkontrastverfahren und seine Anwendung in der Mikroskopie,” 29 (1941), 49-61, written with W. Loos.

Köhler contributed three essays to Handbuch der biologischen Arbeitsmethoden, pts. 1-13 (Berlin-Vienna, 1921-1927): “Das Mikroskop und seine Anwendung,” pt. 1, sec. 2 (1925), 171-352; “Die Verwendung des Polarisationsmikroskops für biologische Untersuchungen,” pt. 2, sec.2 (1928), 907-1108; & Mikrophotographie,” pt. 2, sec. 2 (1931), 1691-1978. His suggestion for making ultraviolet microscopy quantitative will be found in “Mikroskopische Untersuchungen einiger Augenmedien mit ultraviolettem und mit polarisiertem Licht,” in Archiv für Augenheilkunde, 99 (1928), 263-280, written with A. F. Togby.

II. Secondary Literature. For personal details, a full bibliography, and photography, see G. G. Reinert’s obituary notice in Mikroskopie. 4 (1949), 65-70. On Köhler’s scientific contributions the best accounts are by K. Michel, “August Köhler siebzig Jahre alt,” in Naturwissenschaften, 24 (1936), 145-150; and by M. von Rohr, “Persönliche Erinnerungen an A. Köhler,” in Zeitschrift für Instrumentenkunde, 56 (1936), 93-97. Köhler’s work is discussed in F. Schomerus, Geschichte der Jenaer Zeisswerkes 1846-1946 (Stuttgart, 1952), pp. 76-79. Further obituary notices are mentioned in Poggendorff, VIIa, pt. 2, 830-831.

On the way in which Köhler’s ultraviolet microscope was developed, see T. Caspersson’s essay, “Ueber den chemischen Aufbau der Strukturen des Zellkernes,” in Skandinavisches Archiv für Physiologie, 73 , supp. 8 (1936), 1—151.

Robert Olby