Original: Pickering estimated the diameter at 38 miles (61 km), but since he also gave 42 miles (68 km) as the diameter of Phoebe, he was clearly overestimating the albedo; using the modern figure for Phoebe gives Themis a diameter of 200 km.
Simplified: Pickering thought that the diameter of Themis was about 38 miles (61 km). Pickering had also discovered the moon Phoebe, but modern astronomers know that Pickering made a mistake when he gave 42 miles (68 km) as the diameter of Phoebe.