These two volumes include a series of original photographs of primarily the title pages of rare medical books in the National Medical Library pasted onto specially printed sheets with manuscript entries indicating their accession number at the NLM and their number in the sequence of images in the two volumes. Captions for the images are provided in printed lists in the preliminary leaves of each volume. Both volumes have printed title pages and give the impression of being privately printed books, vol. 1 including photographs 1-54, and vol. 2 photographs 55-104. Whether multiple copies of this set were distributed to other institutions after the exhibition was unclear. The selection of the works photographed for this set provides an indication of works regarded by the NLM as major classics at the time.
The first volume contained an explanatory statement by Billings in the form of a letter to General J. K. Barnes dated May 6, 1876 as follows:
"The photographs herewith submitted are intended to indicate some of the older and rare books in the National Medical Library, and are to be placed in the International Exhibition of 1876, in connection with the Catalogues of the Library, as the safest and most convenient means of showing what has been accomplished in the attempt made by this Department to form a collection of medical literature which should meet the demands of the physicians of the United States.
"The photographs have been prepared under the supervision of Assistant-Surgeon J. J. Woodward, U.S.A., and are unusually successful. It must be remembered that many of these books are yellow with age, and that all yellow spots or stains appear in the photograph as dark or black spots.
"The photographs may be considered as being arranged in three series.
"The first series is taken from early printed books, the so-called "Incunabulae," and from other works, which are are rare, at least in this country.
"The second series relates to Surgery, and especially Military Surgery; and the third series is from rare books and pamphlets connected with American Medical History.
"Very respectfully, your obedient servant,
John S. Billings,
Assistant-Surgeon, United States Army
In Charge of Library"
Digital facsimile of both volumes from U.S. National Library of Medicine at this link.