101.  Money Value of Light.  Light is bought and sold almost as readily as are the products of farm and dairy; many factories, churches, and apartments pay a definite sum for electric light of a standard strength, and naturally full value is desired.  An instrument for measuring the strength of a light is called a photometer, and there are many different varieties, just as there are varieties of scales which measure household articles.  One light-measuring scale depends upon the law that the intensity of illumination decreases with the square of the distance of the object from the light.  Suppose we wish to measure the strength of the electric light bulbs in our homes, in order to see whether we are getting the specified illumination.  In front of a screen place a black rod (Fig. 58) which is illuminated by two different lights; namely, a standard candle and an incandescent bulb whose strength is to be measured.  Two shadows of the rod will fall on the screen, one caused by the candle and the other caused by the incandescent light.  The shadow due to the latter source is not so dark as that due to the candle.  Now let the incandescent light be moved away from the screen until the two shadows are of equal darkness.  If the incandescent light is four times as far away from the screen as the candle, and the shadows are equal, we know, by Section 100, that its strength is sixteen candle power.  If the incandescent light is four times as far away from the screen as the candle is, its power must be sixteen times as great, and we know the company is furnishing the standard amount of light for a sixteen candle power electric bulb.  If, however, the bulb must be moved nearer to the rod in order that the two shadows may be similar then the light given by the bulb is less than sixteen candle power, and less than that due the consumer.

[Illustration:  FIG. 58.—­The two shadows are equally dark.]

102.  How Light Travels.  We never expect to see around a corner, and if we wish to see through pinholes in three separate pieces of cardboard, we place the cardboards so that the three holes are in a straight line.  When sunlight enters a dark room through a small opening, the dust particles dancing in the sun show a straight ray.  If a hole is made in a card, and the card is held in front of a light, the card casts a shadow, in the center of which is a bright spot.  The light, the hole, and the bright spot are all in the same straight line.  These simple observations lead us to think that light travels in a straight line.

[Illustration:  FIG. 59.—­The candle cannot be seen unless the three pinholes are in a strait line.]

We can always tell the direction from which light comes, either by the shadow cast or by the bright spot formed when an opening occurs in the opaque object casting the shadow.  If the shadow of a tree falls towards the west, we know the sun must be in the cast; if a bright spot is on the floor, we can easily locate the light whose rays stream through an opening and form the bright spot.  We know that light travels in a straight line, and following the path of the beam which comes to our eyes, we are sure to locate the light.