How did Henry Cavendish use a telescope during the Cavendish experiment?

In the Cavendish experiment, Henry Cavendish used a telescope to observe the torsion balance from outside the sealed room where the apparatus was housed.1

Because the gravitational force being measured was incredibly small—approximately 1/50,000,000 of the weight of the lead balls—Cavendish had to eliminate any potential disturbances, such as air currents or temperature fluctuations caused by his own presence. By observing the balance through a telescope from a distance, he could monitor the movement of the rod as it was drawn aside by the gravitational attraction between the lead spheres without interfering with the experiment. This allowed him to accurately measure how far the rod moved and the duration of its motion, which were essential data points for calculating the density of the Earth.12