Early Observations

The first known observation of the diffuse interstellar features was made by Cannon about 10 years after Hartmann's first observation of interstellar Ca II and Na I, some time between 1911 and 1919 (Code, 1958). The diffuse feature she observed was at a wavelength of approximately 4430$ and was misidentified as stellar H gamma in the spectrum of HD80077 during the preparation of the Henry Draper Catalogue. Some years later, Heger (1921, 1922) observed two absorption lines with aproximate wavelengths of 5780$ and 5797$ in the spectra of six early-type stars, and noted the possibility of their being "stationary" in behaviour similar to the lines of Ca II (H and K) and Na I (D1 and D 2 ) . Wright (1921) also observed a probably stationary line at 6268$, but considered it to be of atmospheric origin. At this time the interstellar origin of the stationary lines of calcium and sodium was still being debated within the astronomical community.

Although observed in the spectrum of HD183143 by Merrill (1930), the lines at 5780$, 5797$, 6284$ and 6614$ were not considered to be of interstellar origin by him until 1934. On the basis of their repeated occurence in early-type stars, Merrill (1934) reported their possible interstellar origin and their "rather diffuse" appearance in comparison with interstellar atomic lines. Two years later Merrill (1936a, 1936b) reported more rigourous tests of the interstellar origin for the above diffuse features. Merrill (1936a) reported their stationary behaviour in the spectrum of the spectroscopic binary HD224151, and the detection of a possible feature at 4427$. The new line was very broad and shallow, and was reported as being "a rather vague feature". Merrill (1936b) reported the behaviour of these features in a small sample of stars of differing radial velocities. Again, the diffuse features displayed radial velocities more concordant with the stationary sodium lines than with the respective stellar spectra. Similar tests were carried

out for the feature at 4430$ by Beals & Blanchet (1937) on a sample of 46 stars of spectral types O and B. The sample included the spectroscopic binary HD216014, and the tests were consistent with an interstellar origin of this feature. A further confirmation of this result was given by Merrill & Humason

(1938).

The first comprehensive survey of the known interstellar lines, including diffuse features, was reported by Merrill et a l .

(1937). This survey was compiled at the Mt. Wilson Observatory using the Cassegrain spectrographs of the 100 inch and 60 inch telescopes. The survey covered a total of 400 stars, with a varying degree of completeness, in both the blue and yellow regions of the spectrum. The yellow spectra were mostly of 34$/mm reciprocal dispersion, and the blue spectra were mostly of 24$/mm reciprocal dispersion. Most stars observed were in the constellations of Cygnus, Perseus, Cepheus or Cassiopeia, and the strength of the 6284$ diffuse feature was measured for only 129 of these stars.

A careful appraisal of the data acquired in the above survey was reported by Merrill & Wilson (1938). It is interesting to note that they also reported the identification of two further possible diffuse interstellar features at 6203$ and 6270$. In their appraisal Merrill & Wilson noted that the diffuse features are normally symmetrical in shape, with only a few noted exceptions. They considered the ratios between the equivalent widths of these lines to be constant within their assigned errors of measurement. Choosing the strongest diffuse feature to be 6284$, Merrill & Wilson then went on to correlate the equivalent width of this line with those of the sodium and calcium lines. They found approximately the same correlation between 6284$ and each of the two atomic species. They further explored the correlation of the equivalent widths of both the 6284$ diffuse feature and the sodium lines with reddening, using colour excesses derived from data given by Stebbins & Huffer (1934). They noted a higher correlation coefficient for the diffuse feature strength and reddening than for the sodium lines and reddening and they observed that this may in part be due to saturation of the sodium lines. In their discussion of the implications of these observations, Merrill & Wilson were the

first to attempt a systematic approach to identifying the cause of the diffuse features. Considering the strengths of the diffuse features, they argued that only strong atomic lines (namely those of sodium and calcium) had so far been identified in the interstellar medium. Considering the well resolved width of these absorption features, they argued against an atomic origin since neither natural nor thermal line broadening mechanisms produced acceptable values for the damping constant and kinetic temperature respectively. A molecular origin was discussed, but proved inconclusive. They noted that the high degree of correlation between diffuse feature strength and reddening implied a closer relationship with the interstellar dust than that between the atomic lines and the interstellar dust. Although no further conclusions could be drawn from this correlation, they also considered the possibility either of a dust origin for the diffuse features or of an origin arising from a dust-gas interaction of some kind.

The discussion of the source of the diffuse interstellar features prompted Swings & Rosenfeld (1937) to suggest the existence of interstellar molecules, in particular the radical CH; Swings & Rosenfeld (1937) noted a weak absorption line at 4300$ observed by Dunham (1937) which they attributed to CH. Eyster (1937) argued strongly against the existence of molecules in the diffuse interstellar medium on the basis of the hostility of the environment to simple molecules. However, soon afterwards McKellar (1940) reported the identification of several sharp interstellar lines as being caused by interstellar CH and CN. Hence, the early work on the diffuse interstellar features probably stimulated the search for and later identification of interstellar molecules. It is interesting to note that, fifty years since Merrill & Wilson reported their study of the diffuse interstellar features, the question of the origin of these features is no clearer. This is despite considerable research into the theoretical, laboratory and observational aspects of the problem.