is countless, and thus from every sensible point of the surface comes, not the original parallel pencil of rays, but a cone of divergent rays. The body itself thus behaves like a candle, or has become luminous, though only by reflected light; and thus it is that if we collect and converge these new cones of rays, either by the eye, or by any other methods, we form an image.

FIG. 31.-Reflection from Unpolished Surfaces.

27. Light Invisible. We push our experiments on scattered reflection a step further, for it bears on a very important matter. Already we cannot help asking ourselves, what is this Light, which obeys rigidly such simple laws, and yet produces such various effects by them? The equal angles of reflection and incidence almost irresistibly suggest to us a ball rebounding from a wall, or a billiard-ball from a cushion, which (except for the influence of any "twist" in the latter case) obey the same law. It is natural to conceive of Light as consisting of infinitely small and highly elastic particles, propelled from the original luminous source; and such a hypothesis will account for most of what we have found, if not all. Being very simple and convenient, and so easily understood, we may therefore provisionally adopt it; but besides some other difficulties we must grapple with later on, the hypothesis has one obvious difficulty that

encounters us even now. Light ought, if it be as supposed, to be visible. And indeed we are apt to picture it as possessing intrinsic brilliancy of its own, and we have even appeared in many of the previous experiments to "see" the course of the rays-the very "rays" themselves—in our darkened room.

Nevertheless it is not so, as a careful consideration of our last experiments soon leads us to perceive. This Light we are studying is not itself a Thing, but a Revealer of things. It is itself, and by itself, absolutely invisible. It makes visible to us luminous objects or sources, rays from which actually reach our eyes; but if we look “sideways" at rays from the most dazzling light, we cannot see them. Space is black. If we appear in previous experiments to have "seen" the course of the rays in our darkened room, this is only because of the little motes in the air; and Professor Tyndall has shown that, destroying these by heat, and keeping fresh ones out of a glass tube thus cleared, the space traversed by the full beam of an electric lamp is dark as night. We demonstrate this less perfectly, but sufficiently, as in Fig. 32. Place on the table a confectioner's glass jar, a, 6 inches in diameter, cover it with a glass plate, B, and drop into it a bit of smoking touch-paper, which soon fills the jar with smoke. Adjust the plane reflector, C, to throw the whole light down as parallel as possible, when the jar is at once filled with a peculiar lambent light. Take off the plate and let the smoke out; and, as it disappears, dark spaces appear where there are no particles to reflect the light, till all is dark. The Light itself is there alike at all times; but where there are no solid particles to reflect it actually to the eye, we see nothing at all. The Light that illuminated the jar is itself invisible.

Once more: clear the jar and fill it with clean water. Again it is almost invisible, except where the rays may be reflected from some point of the glass direct to the eye; and it would

be quite invisible were the clearness of the water and polish of the glass perfect, as we have seen (§ 26). But now pour in one or two spoonfuls of milk and stir it up. At once a splendid opal light fills the jar, and a pleasant radiance the room.

Many students, and even teachers, too much despise these more simple experiments; but they are not only of great beauty, they are pregnant with meaning. We have here not only a difficulty in the "emission" theory which we must not forget, but we have had another striking example of scattered

reflection-that kind of reflection by which most bodies are seen. In white light, more or less of such scattered light is always white. Therefore "coloured" bodies reflect white light as well as coloured; and more white light will be reflected from a black hat in the light, than from a shirt-front in the shade. Smoke is soot, and we all know soot is black; but in our last experiment but one, the light we got reflected from our particles of smoke, when diffused, was white.

CHAPTER III.

REFRACTION.-TOTAL REFLECTION.-PRISMS AND LENSES.

The Refraction or Bending of Rays-The Law of Sines-Index of Refraction-Total Reflection-The Lu Dinous Cascade-PrismsLenses-Images produced by Lenses-Focus of a Lens-Virtua Images and Foci.

28. Refraction.-Provide a rectangular tank about two inches between the sides, one of which, to serve as the front is a piece of glass a foot square, or a little more; let one end also be of glass, and the top open.1 Paint over the face with black varnish all but a circle, on which paint a horizontal and perpendicular line through the centre, as in Fig. 33. Provide also a strip of thin zinc or copper blackened, CD, rather wider than the tank, and about three inches longer, in which cut two slits inch wide, and nearly the whole width of the strip (or depth from front to back of the tank) in length, in such positions that when the strip rests perpendicularly against the glass end, the slit E shall be about inch above the horizontal line, and the slit F make an

1 Professor Tyndall was the first to employ this striking method for lantern demonstration of refraction. His tank was circular, like a clockface. The square form here described is more easily made, and the loose cover and slits have advantages in demonstrating total reflection.

E

angle of 40 degrees from the centre of the circle with the horizontal line.

FIG. 33.

Fill the tank exactly to the horizontal line with water mixed with two or three drops only of milk; place the metal strip over the top with both slits towards the lantern, and arrange the reflector as in Fig. 34, placing in the optical

stage the slit used in our first experiment in reflection, horizontally, or using it with parallel light. First of all direct the light through the slit E (Fig. 33), only a little off the