Light issues from the sun, passes through millions of miles to the earth, and as it falls upon different substances, a variety of effects are apparent. There is a certain class of bodies which obstruct the passage of the rays of light, and where light is not, a shadow is cast, and the substance producing the shadow is said to be opaque. Wood, stone, the metals, charcoal, are all examples of opacity; whilst glass, talc, and horn allow a certain number of the rays to travel through their particles, and are therefore called transparent. Nature, however, never indulges in sudden extremes, and as no substance is so opaque as not (when reduced in thickness) to allow a certain amount of light to pass through its substance, so, on the other hand, however transparent a body may be, a greater or lesser number of the rays are always stopped, and hence opacity and transparency are regarded as two extremes of a long chain; being connected together by numerous intermediate links, they pass by insensible gradations the one into the other. If a gold leaf, which is about the one two-hundredth part of an inch in thickness, is fixed on a glass plate and held before a light, a green colour is apparent, the gold appearing like a green, semi-transparent substance. When plates of glass are laid one above the other, and the flame of a candle observed through them, the light decreases enormously as the number of glass-plates are increased. Even in the air a considerable portion of light is intercepted. It has been estimated that of the horizontal sunbeams passing through about two hundred miles of air, one two-thousandth part only reaches us, and that no sensible light can penetrate more than seven hundred feet deep into the sea; consequently, the vast depths discovered in laying the Atlantic telegraph must be in absolute darkness. [Illustration: Fig. 253.] Light is thrown out on all sides from a luminous body like the spokes of a cart-wheel, and in the absence of any obstruction, the rays are distributed equally on all sides, diverging like the radii drawn from the centre of a circle. As a natural consequence arising from the divergence of each ray from the other, the intensity of light decreases as the distance from the luminous source increases, and _vice versâ_. Perhaps the best mechanical notion of this law is afforded by an ordinary fan; the point from which the sticks radiate, and where they all meet, may be [Page 266] termed the light; the sticks are the rays proceeding from it. (Fig. 253.)