ARTICLES LAID ON THE TABLE. A GLASS PRISM. RULER, OR STRAIGHT STICK. BASIN OF WATER. ROUND DECANTER, OR GLASS GLOBE, WITH WATER. PLAIN, CONVEX, AND CONCAVE MIRRORS. DECANTER, WITH COLOURED LIQUID, WELL CORKED. 83 LECTURE III. REFRACTION.-LENSES AND MIRRORS. THE variety of useful and ornamental purposes to which glass is applied is so great, that we must this evening confine ourselves to those which are connected with its transparency, and to the effects of certain changes in the direction of the rays of light, which are produced by their passage through pieces of glass of different forms, or by being reflected from them. No doubt you all perceive the convenience set forth by the old chronicler, of "keeping out the birds and the rain, without excluding the light"—of having our houses warm, dry, and cheerful; and, when we travel in a carriage, of being able to enjoy the scenery through which 84 COLOUR A PROPERTY we are passing, in defiance of the weather. And yet these advantages are but a small part of our obligation to the transparency of glass. That property has done wonders in promoting the extension of knowledge—wonders far beyond my ability to explain, or yours to comprehend. Shall I try to give you a faint notion of some of them? I dare say, that in showery weather, you have frequently, when the sun has been hidden by a small, dark cloud, admired its bright rays issuing, as from behind a skreen, and streaming over other clouds in long lines of light. Well, such rays are always proceeding from the sun in straight lines, and spreading in all directions through the air. In the broad light of day we do not usually perceive these rays, we see only the general effect of them in the brightness of every object on which they fall; but if we close the shutters, so as to darken the room, and then make a small hole in the shutter, a ray of light will be seen streaming across the darkness. By means of a bar of glass like this, which is called a prism, I could show you that the ray, which, when we look at it with our unassisted eyes, appears of one white colour, is in reality composed of all the colours of the rainbow. When light passes through a piece of glass of this shape, it is separated into those distinct colours; and, if we hold a sheet of white paper at a proper distance behind the prism, we shall see the colours of the rainbow, in their natural order, reflected upon it. It is to this beautiful experi ment that Dr. Darwin alludes, when he says, "Cling round the aerial bow with prisms bright, And, pleased, untwist the sevenfold threads of light." Here, you perceive that he compares a ray of light to a thread of seven colours twined together; and it is a very just comparison. But perhaps we might never have known it to be just-never have discovered that the greenness of the grass and trees, and the beautiful colours of the flowers in the garden, are not in the grass and flowers themselves, but that the colours we admire are properties of the light which falls upon those objects, and is reflected from them. |