Yerkes Observatory Log: Barnard and the Bruce Camera
Edward Emerson Barnard adopted the Bruce camera as his special instrument. Over the next twenty years, he spent countless hours from dusk until dawn photographing sections of the Milky Way. These photos revealed many dark clouds ("coal sacks") within the Milky Way. Many years before, Herschel had also seen them visually and concluded that: "Surely this is a hole in the heavens!" However, Barnard was one of the first to realize that these were actually great clouds of dust that obscured our view of the stars beyond. It was obvious that if this were dust, then the universe is not as empty as had been supposed. But the nature of this dust remained a mystery for Barnard. He considered that it might be either the remains of long-dead stars or elementary particles that might one day become gravitationally bound and ignite to form new stars. The prevailing view is that this dust may be tied to both sources, though most is probably from the latter -- often referred-to as "primordial matter". It was left for Bart Bok to interpret these dark spots and identify the different types, some of which now bear the nickname "Bok Globules". Today many of the dark nebulae have been identified with active star-forming regions.
The fruit of this work is seen on our more detailed atlases which identify these dark, obscuring clouds as "Barnard Objects" (the letter B followed by a number). They're visible in a dark sky, and are particularly abundant in the heart of the Milky Way near Sagittarius. Take a look for them with binoculars or a telescope with a wide field eyepiece. Their presence is detected by areas where there are conspicuously fewer stars.
As a side note, George W. Ritchey helped the effort to photograph the heavens by inventing a double-slide plate holder for the cameras used at Yerkes. This improved the accuracy of tracking by allowing the photographic plate to be moved in a framework by means of handscrews, rather than relying strictly on movement of the entire telescope. This device soon became standard equipment at all observatories.