The planet Mercury did not receive much attention from Herschel; but, slight though his interest in it seems to have been, he could not make it a field of observation without shedding light on things then unknown, and afterwards forgotten. As a transit or passage of the planet over the sun's face was due at Windsor in the early morning of November 9, 1802, and Herschel's "apparatus[1] for viewing the sun was then in the highest perfection," he was on the watch for what might happen. The weather proved as favourable as he could wish, and more than forty dark spots were counted on the sun's disc. A little black pea traversing the disc among dark spots of vastly greater size, it might have been feared, would be lost to view or only seen now and again. On the contrary, the black dot was easily seen during the four hours that remained of its passage across. As the sun rose higher, "the corrugations of the luminous solar surface up to the very edge of the planet" were visible with a 10-feet reflector. "When the planet was sufficiently advanced towards the largest opening," or spot, "of the northern zone, he compared the intensity of the blackness of the two objects; and found the disk of Mercury considerably darker, and of a more uniform black tint, than the area of the large opening." As it approached the edge of the sun, the whole of its disc was "as sharply defined as possible; there was not the least appearance of any atmospheric ring, or different tinge of light, visible about the planet." As the black dot vanished on leaving the bright body of the sun, there was not the slightest distortion of the sun's limb or in its own figure. The
- ↑ The mirror of the reflector used on this occasion was made of glass.
