At the historical archive of the Pontifical Gregorian University, in the Secchi fund, a substantial number of documents is preserved, the main nucleus of which is represented by the correspondence of the Jesuit scientist, for the most part unpublished. It can be subdivided as follows:
- Scientific correspondence: About 7,000 letters from over 1,500 correspondents;
- Correspondence in the Jesuit field: about 700 letters exchanged with Jesuit confreres, which in some cases still present a scientific content;
- Personal correspondence: About 200 letters identifiable in various forties of the Fund, consisting of personal and family correspondence or related to the activities of the various scientific societies of which Secchi was a member.
The letter reproduced here is part of the scientific correspondence. The sender is the famous English astronomer John Herschel, whom Secchi met in person during one of his trips to England. In the letter, Herschel shows appreciation for the Meteorologic Bulletin of the Roman College Observatory, of which it received the first number. In particular, he focuses on the article containing the description of the thermo-Heliometer, which he compares with his actinometer, a tool with which Herschel was among the first to attempt to measure the amount of heat irradiated by the sun. The fund contains three more letters from Herschel written between 1861 and 1869.