As part of their ongoing study of sunspots during the academic terms and using a method she had devised and equipment obtained with her personal funds, Maria Mitchell and her students photographed the sun on most clear days between 1877 and her retirement in 1888. Meticulously annotating, as was her custom, a photograph of sunspots taken on May 29, 1880, Mitchell noted, "No photographs have been taken since May 24 on account of the intense heat, the thermometer being 90º and 92º."

More that 100 years after Mitchell's death in 1889, over 800 of her glass negatives were found, along with her notations, on hand-dated shelves in a shallow closet in the Observatory.  They are now preserved in Vassar's Special Collections Library.