The large telescope at Meudon has become legendary. When it was conceived, after 1870, astronomy as a whole was limited to visual observation. Knowledge of the sky was limited to what one could see, assisted only by optical means. The large telescopes produced at this time produced larger images, permitting close-up views: the Meudon telescope was able to accomplish this perfectly.

At Meudon, which became the Mecca of visual observation, the major planets were examined in a way that no other telescope had previously been able to. The telescope monitored the state of their atmospheres and...