2.1. Interstellar Dust: Early Observational Evidences
"Surely, there is a hole in the heavens!"
-- Sir William Herschel 
The Milky Way looks patchy with stars unevenly distributed: looking at the sky in the direction of Sagittarius it is clear that there are tremendously dark lanes especially in the region toward the Galactic Center. The subject of these dark patches and what makes them dark also have a very patchy history. The existence of dark regions in the Milky Way was first pointed out by Sir William Herschel in the late 18th century. At that time, these dark lanes - the dust clouds which obscure the light from the background stars, were considered as "holes in the heavens" (Herschel 1785).
"They are really Obscuring bodies!"
-- Agnes Clerke 
In August 1889, Edward Barnard started to take pictures and reported vast and wonderful cloud forms with their remarkable structure, lanes, holes and black gaps. At the beginning of the 20th century, astronomers started to realize that they "were really obscuring bodies" rather than holes devoid of stars (Barnard 1919). Agnes Clerke (1903) stated in an astrophysics text that "... The fact is a general one, that in all the forest of the universe there are glades and clearings. How they come to be thus diversified we cannot pretend to say; but we can see that the peculiarity is structural - that it is an outcome of the fundamental laws governing the distribution of cosmic matter. Hence the futility of trying to explain its origin, as a consequence, for instance, of the stoppage of light by the interposition of obscure bodies, or aggregations of bodies, invisibly thronging space."
Heber D. Curtis and Harlow Shapley (3) held a famous debate in 1920 (Shapley & Curtis 1921); among the points of contention was whether what is seen as the dark lanes in the Milky Way is caused by obscuring material. Curtis said the dark lanes observed in our Galaxy were obscuring material, while Shapley said he found no evidence of obscuring material in his observations of globular clusters. Later observers became aware that Shapley's argument was irrelevant because the globular clusters are out of the plane of the Galaxy. The obscuring dust was confined to the so called "plane of avoidance" which is the Galactic plane.
"Stars are dimmed!"
-- Wilhelm Struve 
The presence of interstellar extinction was pointed out as early as in 1847 by F.G. Wilhelm Struve. He found that the number of stars per unit volume seems to diminish in all directions receding from the Sun. This could be explained either if the Sun was at the center of a true stellar condensation, or if the effect was only an apparent one due to absorption (which may have been understood to include light scattering). He argued that there could be an visual extinction of about 1 mag kpc-1 in interstellar space.
Jacobus C. Kapteyn (1904) had found a roughly spherical distribution of stars around the Sun. He assumed a constant stellar density and then used the observed density to arrive at a value for the extinction (absorption) of light 1.6 mag kpc-1 (Kapteyn 1909), which differs little from current values ( 1.8 mag kpc-1 assuming a hydrogen density of nH = 1 cm-3). (4)
In 1929 Schalén examined the question of stellar densities as a function of distance. He did a very detailed study of B and A stars, including those in Cygnus, Cepheus, Cassiopeia, and Auriga. He obtained rather different values of the absorption coefficient, particularly in Cygnus and Auriga where there are large dark patches. So obviously the absorption is more in some regions and less in others.
"Cosmic dust particles produce the selective absorption."
-- Robert J. Trumpler 
It was not until the work of Robert J. Trumpler in 1930 that the first evidence for interstellar reddening was found. Trumpler (1930) based this on his study of open clusters in which he compared the luminosities and distances of open clusters with the distances obtained by assuming that all their diameters were the same. By observing the luminosities and knowing the spectral distribution of stars he was able to find both absorption ( 0.7mag kpc-1) and selective absorption or color excess (between photographic and visual; 0.3 mag kpc-1) with increasing distance, and produce a reddening curve. (5) It was this work which led to the general establishment of the existence of interstellar dust.
The wavelength dependence of extinction in the optical was measured for the first time by Rudnick (1936) using the still-widely-used "pair-match" method. Further observations carried out by Hall (1937) and Stebbins, Huffer & Whitford (1939) pointed to an -1 reddening "law" (at that time limited to 1-3 µm-1): the reddening curve showed a rise inversely proportional to the wavelength .
In 1934 Paul W. Merrill reported the discovery of the 5780 Å, 5797 Å, 6284 Å, 6614 Å "unidentified interstellar lines". These widened absorption lines, now known as "Diffuse Interstellar Bands", still remain unidentified (see Krelowski 2002 for a review).
The existence of diffuse interstellar radiation, originally detected by van Rhijn (1921), was verified and attributed to small dust grains by Henyey & Greenstein (1941). Henyey & Greenstein (1941) found that interstellar particles are strongly forward scattering and have a high albedo.
"Starlight is polarized!"
-- John S. Hall ; W.A. Hiltner 
At the end of 1940s, two investigators (Hall 1949; Hiltner 1949), inspired by a prediction of Chandrasekhar on intrinsic stellar polarization, independently discovered instead the general interstellar linear polarization. Magnetic fields were believed to confine cosmic rays and to play a role in the spiral structure of the Galaxy. The implication of the linear polarization was that the extinction was caused by non-spherical particles aligned by magnetic fields (Davis & Greenstein 1951). The wavelength dependent polarization curve was later shown to be well represented by the Serkowski law, an empirical formula (Serkowski 1973). In addition to the extinction curve which was later also extended to a wide wavelength range, the polarization law as well as the polarization to extinction ratio provide further insight into the physical and chemical nature of interstellar dust.
The circular polarization produced by interstellar birefringence (Martin 1972) was originally predicted by van de Hulst (1957). It was first detected along the lines of sight to the Crab Nebula by Martin, Illing, & Angel (1972) and to six early-type stars by Kemp & Wolstencroft (1972).
3 Henry Norris Russell (1922), Shapley's advisor at Princeton, believed that the existence of dark clouds accounted for the obscuration and argued that this obscuring matter had to be the form of fine dust. But Shapley did not follow his advice. Back.
4 But Kapteyn did not take this seriously; for example, he assumed no extinction in his grand 1922 paper on the motion of stars in the Galaxy (Kapteyn 1922). Back.
5 Trumpler's observations indicated reddening even where he saw no clouds. Dufay (1957) questioned whether interstellar space outside dark clouds and nebulae should be considered perfectly transparent. Back.