Again, it is seen by Fig. 225 that if the screen be placed at the distance of three feet the intensity of the light is one-sixteenth of that which it is at the pasteboard. While the distances, therefore, are as 1, 2, 3, 4, etc., the intensity of the light is inversely as the numbers 1, 4, 9, 16, etc., that is inversely as the squares of the distance.
338. Velocity of Light.—The velocity of light is so great that within any ordinary distances it may be considered as instantaneous. Thus when we measure the distance of a cannon by the difference between the time of its flash and the report, we do not reckon the light to consume any time in its passage to the eye. But when we come to look at objects as distant as the sun and other heavenly bodies, we reckon in our calculations the time of the passage of light. It takes light eight minutes to travel from the sun to us, a distance of ninety-five millions of miles. With the telescope stars have been seen which have been ascertained to be at such a distance that it requires over ten years for their light to come to the earth. Others have been seen which are much farther off, but their distances have not been absolutely ascertained. Some have been seen supposed to be at such a distance that the light coming from them to the eye of the astronomer was a hundred thousand years in its passage.
Fig. 226.
339. Roemer's Observations.—The velocity of light was first determined by Roemer, a Danish astronomer, in 1676. It was done in his calculations and observations of the eclipse of one of Jupiter's moons. After making the calculation of the time it would take for the satellite to pass through the shadow of the planet, he observed its passage, and found that it did not come out from the shadow as soon as his calculation required by fifteen seconds. What was the difficulty? If the earth had remained in one spot from the beginning to the end of the passage of the satellite, the observation would have come out exactly according to the calculation. But the earth had moved during this time (about forty-two hours and a half) the immense distance of 2,880,000 miles. The light of the emerging satellite therefore had to travel over this additional distance to overtake the earth, and it took fifteen seconds to do it. If we divide, then, this distance by 15 we get the distance which light travels in a second, which is 192,000 miles. All this can be made clear by the diagram, Fig. 226. Let S be the sun, J Jupiter, and C one of its moons emerging from its shadow. Let A be the earth as it is when the eclipse of Jupiter's moon begins. When it emerges the earth has passed to B, and the light from the satellite has to travel as much farther to reach it now as B C is longer than A C. Roemer made other observations with the earth at some other parts of her orbit with the same result.
Fig. 227.
340. Reflection of Light.—Light, like sound and heat, is reflected in straight lines when it strikes upon any resisting substance. We can see this to be the case when it strikes upon any smooth and plane surface. And it is true of light, as it is of heat, that the angles of incidence and reflection are equal. Thus if c, Fig. 227, be a reflecting surface, and b c a line perpendicular to it, then a ray of light, d c, will be reflected in the line c a, and the angle of incidence, d c b, will be equal to the angle of reflection, b c a.
341. How we See.—We see the various objects around us by the light which is reflected from them. Every point of every surface that we see reflects rays or vibrations of light to our eyes. Thus if we see a person there are rays of light reflected into our eyes from every part of him. These rays form an image of him in the back part of each eye, and it is by this image that we see him, as will be explained in full in another part of this chapter. Reflected light is painting the images of objects in the eye every moment in great abundance and variety. If a speaker have an audience of a thousand persons all looking at him, his image is at the same time in two thousand eyes, and in each of these two thousand images every motion and every changing expression are faithfully depicted.
