The Discovery of Neptune
The mathematicians John Couch Adams of England and Urbain Jean Joseph Leverrier of France both individually concluded that there should be another planet in the solar system using calculations based on the orbit of Uranus. Adams attempted to confirm his hypothesis by contacting the Royal Observatory and Cambridge, but both were unsuccessful. At the same time, Leverrier contacted Johann Gottfried Galle of the Berlin Observatory. Using their refracting telescope, Galle and his student Louis d'Arrest discovered Neptune on 23rd September 1846.
Sources:
https://www.rmg.co.uk/stories/topics/astronomy-19th-century