The Scientists Collection is comprised of individual letters and small groups of correspondence from American, British, French, and German scientists during the past three centuries. Although the content is highly varied, there is significant strength in astronomy, natural history, conchology, and geology. Among the scientists better represented in the collection are the astronomers William Radcliffe Birt, J.F.W. Herschel, and Franz Xaver von Zach; the conchologists A.D. Brown, Fred L. Button, Otto Mörch, Alfred Newton, Christian M. Poulsen, Temple Prime, Benjamin Lewis Vulliamy, and A. G. Wetherby; the physical scientists George Biddell Airy, Arnold Sommerfeld, Ferdinand R. Hassler, and Max Planck; the archaeologist Jean François Nadaillac; the philosopher William Whewell; and the naturalists Walter Henry Bates, Robert Chambers, Edme Dupuget, Robert Kaye Greville, Joseph Henry, John Stevens Henslow, John Lubbock, and Herbert Spencer.

These are primarily letters to Gray on various aspects of natural history, with many references to collections at the British Museum of Natural History. There are numerous letters from British scientists and from European colleagues. There are letters from Americans too, such as those from Agassiz which relate to his Florida expeditions and the creation of natural history collections.