This book presents a detailed analysis of some highly significant astronomical instruments designed in the German states during the 19th century: Carl Steinheil's astronomical drawing-board, Karl Friedrich Zöllner's astro-photometer and Oswald Lohse's astro-camera. Based on these instruments, the author shows how, by rebuilding the instruments and practising with them, significant historical insights can be gained. The author demonstrates how observers interacted with new instrument designs and how different observers using new types of instruments could reach agreement. The newly founded Astronomische Gesellschaft enabled a rapid distribution of these new instruments together with the communication of observations. This book argues that in the new discipline of astro-photometry, scientific practice eventually overcame theoretical doubts.