If the "Digital Rare Book Collection" at the Vienna University Observatory was helpful for your work, the following acknowledgment would be appreciated:
This work has made use of the "Digital Rare Book Collection" at the Vienna University Observatory, located at http://hwastro.univie.ac.at
A.Univ.Prof.Dr. Franz Kerschbaum - Institut für Astronomie der Universität Wien
DDr. Thomas Posch - Institut für Astronomie der Universität Wien