If, while sitting in a room, you look earnestly at the middle of a window, a little while, when the day is bright, and then shut your eyes, the figure of the window will still remain in your eye, and so distinct that you may count the panes. A remarkable circumstance attending this experiment is, that the impression of forms is better retained than that of colours; for, after the eyes are shut, when you first discern the image of the window, the panes appear dark, and the cross-bars of the sashes, with the window frames and walls, appear white and bright; but if you still add to the darkness of the eyes, by covering them with your hand, the reverse instantly takes place--the panes appear luminous, and the cross-bars dark; and by removing the hand, they are again reversed. _Curious Effects of Oil upon Water, and Water upon Oil._ Fasten a piece of pack-thread round a tumbler, with strings of the same from each side, meeting above it in a knot at about a foot distance from the top of the tumbler. Then putting in as much water as will fill about one-third part of the tumbler, lift it up by the knot, and swing it to and fro in the air; the water will keep its place as steadily in the glass as if it were ice. But pour gently in upon the water about as much oil, and then again swing it in the air as before, the tranquillity before possessed by the water will be transferred to the surface of the oil, and the water under it will be violently agitated. _Another curious Experiment with Oil and Water._ Drop a small quantity of oil into water agitated by the wind; it will immediately spread itself with surprising swiftness upon the surface, and the oil, though scarcely more than a tea-spoonful, will produce an instant calm over a space several yards square. It should be done on the windward side of the pond or river, and you will observe it extend to the size of nearly half an acre, making it appear as smooth as a looking-glass. One remarkable circumstance in this experiment is the sudden, wide, and forcible spreading of a drop of oil on the surface of the water; for if a drop of oil be put upon a highly polished marble table, or a looking-glass, laid horizontally, the drop remains in its place, spreading very little, but when dropped on water it spreads instantly many feet round, becoming so thin as to produce the prismatic colours for a considerable space, and beyond them so much thinner as to be invisible, except in its effect in smoothing the waves at a much greater distance. It seems as if a repulsion of its particles took place as soon as it touched the water, and so strong as to act on other bodies swimming on the surface, as straw, leaves, chips, &c., forcing them to recede every way from the drop, as from a centre, leaving a large clear space. _Remarkable Effects on the visual Nerves, by looking through differently-coloured Glasses._