To our knowledge the first time this "Bird's Nest" marking was considered as a possible ring of absorbing nebulosity was in 1910 when the senior author noticed the object and mentioned the possibility that it might be an actual ring of dark absorbing nebulosity. The ring is conspicuously evident upon every exposure of long enough duration to exhibit such markings. It is located in the "Labrador" section of the North America Nebula, right ascension 20 hours, 52 minutes, declination plus 45 degrees. Its external diameter is about thirty minutes of arc and its internal diameter about twelve minutes of arc.
Proceedings of the Iowa Academy of Science
©1928 Iowa Academy of Science, Inc.
Morehouse, D. W. and Zug, Richard S.
"Star Count in the Dark Ring Nebula in Cygnus,"
Proceedings of the Iowa Academy of Science, 35(1), 55-67.
Available at: https://scholarworks.uni.edu/pias/vol35/iss1/7