The records consist almost entirely of observational data gathered by astronomers using Dearborn's 18 1/2 inch refracting telescope or its meridian circle, calculations performed on the data, and the results of such investigations. Also included are six folders of clippings, reports, and miscellaneous correspondence and two boxes of visitors registers and material relating to the observatory's library. Initially all observations were entered sequentially in notebooks, 205 of which survive. Separate records were kept for the equatorial (the 18 1/2 in. refractor) and the meridian circle. Early notebooks are arranged as either equatorial or meridian circle observations and then chronologically. Later notebooks are grouped according to the subject of the observation. The records include five and a half volumes of general observations made with the equatorial from 1863-1908, primarily by directors Stafford, Colbert, and Hough, and some by S.W. Burnham. They include records of planetary studies, with observations of Jupiter, Uranus, and Neptune being the most numerous; steller observations; observations made to establish longitudes and latitudes; and observations of other cellestial objects and events such as astroids, comets, and eclipses. In many cases the data is not clearly identified, thus limiting its usefullness to all but the experienced astronomers. For the most part, no narratives exist detailing the observations for the non-scientist. Sixty volumes of meridian circle observations are arranged according to types of observations and then chronologically within each category. Participation of the observatory in a project directed by the Astronimische Gesellschaft in Berlin is documented from 1868-1971 when the Chicago fire curtailed research. Systematic meridian circle observations began again in 1879. Also included are records relating to instruments used in the observatory. With the directorship of Philip Fox, the notebook system was discontinued and regular reports of observations became the norn. They included observers name, date, nature of observation. The bulk of data from Fox's tenure concerns the stellar parallax measurement project and includes one box of spectroheliograms that detail the sun's features by photographing it with monochromatic light, and five boxes of parallax measurements taken between 1916-1929 arranged chronologically. There are also one and a half boxes of reduction calculations. Several large projects were carried out under Oliver Lee: time clock records, 1933-1934; observations of the asteroid Eros in 1930-1931; the Red Star Project from 1931-1941. The only observatioonal records from Hynek's tenure are 2 oversize folders of charts recording magnitudes of stars. The charts are not clearly labelled and require sophisticated knowledge of astronomy to be useful. An addition includes three notebooks of spectrograph records. The first two of these books contain data for plates no. 1-1958, 1920-1930. The third contains duplicate material, in ink rather than pencil for plates no. 1-1260.