The Trouble with the Instrument: Zöllner's Photometer

Zöllner On Körber see F. , Karl Friedrich Zöllner (Berlin, 1899), and Herrmann D. B. , Karl Friedrich Zöllner (Leipzig, 1982).

Rucks P. , “Prüfung astronomischer Optik mit Laserinterferometrie”, Sterne und Weltraum , xii (1996), 954–9.

Zöllner K. F. , Grundzüge einer allgemeinen Photometrie des Himmels (Berlin, 1861), 14 (my translation).

Nicol W. , “On a method of so far increasing the divergency of the two rays in calcareous-spar, that only one image may be seen at a time”, Edinburgh new philosophical journal , vi (1829), 83–84.

The matching was done by simply photocopying all drawings on foils in the same scale; the different drawings could then be combined easily.

The historians replicating experiments at Oldenburg report that problems related to the replication process can occur as a result of misunderstandings between the historian and the engineer supervising the technical aspects of the work. This is the consequence not only of technical misunderstandings but also of differing social attitudes of both parties, expressed for example in language or outward appearance. The need to have the workshops' engineers ‘on site’ for a successful construction of an instrument was already known in the nineteenth century. Simon Schaffer has pointed this out in his study of Maxwell's work on the replication of scientific experiments: “Late Victorian metrology and its instrumentation: A manufactory of Ohms”, in Bud R. Cozzens R. S. (eds), invisible connections (London, 1992), 23–56, p. 34.

Sibum H. O. , “Working experiments: Bodies, machines and heat values”, in Staley R. (ed). The physics of empire (Cambridge, 1994), 29–56. Sibum writes about the rebuilding of experiments: “The process of reconstructing the objects is potentially an extremely rich resource for an historian of science” (p. 36).

Bunsen R. Roscoe H. , “Photochemische Untersuchungen II”, Poggendorff'sche Annalen der Physik und Chemie , c (1857), 43–89.

Zöllner , op. cit. (ref. 4).

Bruhns Chr. , “Meteorologisches Tagebuch der Sternwarte Babelsberg, Sternwarte Babelsberg, Oktober 1859 — November 1860”, Berlin Brandenburgische Akademie der Wissenschaften, Akademiearchiv, Signatur 679.

For a report of this procedure see: Rosse Lord , “Zöllner's astro-photometer”, Conferences held in connection with the special loan collection of scientific apparatus (London, 1876), 92–94. The specific design of Lord Rosse's photometer indicates that he might have used it for photometry of nebulae.

Another possible error is the interval error. This I had already taken into account: Because of the geometric relationship between the intensity and the turning angle of the Nicol prism, the possible reading error increases when the prisms are turned (being least at 0° and greatest at 90°). However, all my measurements were carried out in the interval 10°–50°, and I could detect no geometric errors in the measurements.

Every measurement was preceded by the selection of two diaphragms, to reduce the aperture of the object lens and the diameter of the artificial star, depending upon the brightness of the stars to be observed. The pressure of the (replicated) town gas was set to 0.5 bar and the pressure of the ethylene to 0.75 bar. I usually waited half an hour after the gas had been lighted to make sure the flame was stable.

Zöllner , op. cit. (ref. 4), 55.

One does find a few stars that Zöllner observed more than once and which enable us to connect at least a few of his measurement sequences. I have tried to reduce Zöllner's measurements for current photometric research on the basis on my own observations. Sterken C. Staubermann K. , “Zöllner's Catalogue of Visual Magnitudes”, in Sterken Staubermann (eds), Karl Friedrich Zöllner and the historic dimensions of astro-photometry (Brussels, 2000), 95–122.