retina. Now "the planet is very faint; and the influence of its feeble light cannot extend far with any degree of equality. This enables us to see the faintest objects, even when they are only a minute or two removed from it. The satellites are very nearly the dimmest objects that can be seen in the heavens; so that they cannot bear any considerable diminution of their light, by a contrast with a more luminous object, without becoming invisible. If then the sphere of illumination of our new planet be limited to 18" or 20", we may fully account for the loss of the satellites when they come within its reach; for they have very little light to lose, and lose it pretty suddenly." This view of a weak light extinguishing a weaker, though a commonplace now, received a very poetical interpretation in a paper written by Herschel three years after. "This increased sensibility," he says, "was such, that if a star of the 3rd magnitude came towards the field of view, I found it necessary to withdraw the eye before its entrance, in order not to injure the delicacy of vision acquired by long continuance in the dark. The transit of large stars, unless where none of the 6th or 7th magnitude could be had, has generally been declined in my sweeps, even with the 20-feet telescope. And I remember, that after a considerable sweep with the 40-feet instrument, the appearance of Sirius announced itself, at a great distance, like the dawn of the morning, and came on by degrees, increasing in brightness, till this brilliant star at last entered the field of view of the telescope, with all the splendour of the rising sun, and forced me to take the eye from that beautiful sight." To increase this sensibility of the eye he was on these occasions in the
