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small a planet from the great number of fixed stars which studded the heavens around it, it was necessary to proceed by determining the position of as many stars as possible several times, in order that, by a comparison of the observations, it could be determined whether any of them had moved out of its place.

While Mr. Challis was engaged in this work it occurred to Leverrier that the astronomers of Berlin were mapping the heavens. He therefore wrote to Encke, the director of the Berlin Observatory, suggesting that they should look for the planet. Now it happened that the Berlin astronomers had just completed a map of that part of the sky in which the planet was located. So, on the very evening after the letter was received, they took the map to the telescope and proceeded to search about to see if any object was seen in the telescope which was not on the map. Such an object was very soon found, and, by comparing its position with that of the stars around it, it seemed to have a slight motion. But Encke was very cautious and waited for the discovery to be confirmed on the night following. Then it was found to have moved so much that no doubt could remain, and he wrote Leverrier that the planet actually existed.

When this news reached England, Professor Challis proceeded to examine his own observations, and found that he had actually observed the planet on two occasions. Unfortunately, however, he had not reduced and compared his observations, and so failed to recognise the object until after it had been seen at Berlin.

The question of the credit due to Adams gave rise to