One of the greatest discoveries made with the Potsdam double refractor was the discovery of the interstellar medium, inferred from the stationary Calcium H and K lines seen in the photographic spectrograms of the single-lined spectroscopic binary star δ Orionis, which is one of the Orion belt stars. The observations were made by Johannes Hartmann, Vogel's assistant, in the winters of 1901/1902 and 1902/1903.

Contrast enhancement and composite by Ulfert Hanschur

The calcium line is in the middle (Scan by UTECH)

One of the original discovery plates of the stationary Calcium K line in the radial velocity spectrogram of δ Orionis taken by Hartmann in 1902. The size of the glass plate is about to scale. The δ Orionis spectrum is sandwiched between two calibration spectra, above and below, and an extraction is shown enhanced on the bottom.

Radial velocity measurements of the orbital motion of the δ Orionis close physical pair (SB1, P=5.7 days) during four nights (2000 January 18, 19, 23, 24). The observations were made with the echelle spectrograph at the Tautenburg 2m telescope in Thuringia, Germany, by E. Guenther. The spectral resolution was R=35000. Note the shifting of the rotationally broadened He I lines (FWHM=215 km/s) near 5876 Å, while the Na D1 and D2 lines at 5890 and 5895 Å remain narrow and "stationary". This demonstrates the presence of neutral Na in the line of sight toward this Orion binary object. From observations similar to these, but using the stationary Ca II H and K lines at 3968 and 3933 Å, Potsdam astronomer Johannes Hartmann in 1904 correctly deduced the presence of an "interstellar medium" for the first time.

The information is based on a paper by Hans Zinnecker "Binary Stars: Historical Milestones" in The Formation of Binary Stars, IAU Symposium Vol. 200, 2001, edited by H. Zinnecker & R.D. Mathieau. Since the book is out of stock, here is the PDF version of the article.