Fig. 235.
The most astonishing instance of mirage of which I have ever heard is thus narrated: "The cliffs on the French coast are 50 miles distant from Hastings, on the coast of Sussex, and they are actually hidden from the eye by the convexity of the earth; that is to say, a straight line drawn from Hastings to Calais or Boulogne would pass through the sea. A year or two ago, however, a Fellow of the Royal Society, who was residing at Hastings, was surprised to see a crowd of people running to the sea-side. Upon inquiry as to the cause of this he was informed that the coast of France could be seen by the naked eye. He immediately went down to the shore to witness so singular a sight, and there discovered distinctly the French cliffs extending for some leagues along the horizon, and so vividly that they appeared to be only a few miles off. The sailors and fishermen, with whom Mr. Latham walked along the water's edge, could hardly at first be persuaded of the reality of the appearance; but as the cliffs gradually became more elevated they were so convinced that they pointed out to Mr. Latham the different places they were accustomed to visit—such as the bay and the wind-mill at Boulogne, St. Vallery, and other places on the coast of Picardy, even as far as Dieppe, all the French shores appearing to the English sailors as if they were sailing at a short distance from them toward the harbors. With the aid of a telescope the French fishing-boats were plainly seen at anchor; and the different colors of the land upon the heights, together with the buildings, were perfectly discernible. The day when this occurred is said to have been extremely hot, without a breath of wind stirring, and the phenomenon continued visible in the highest splendor until past eight o'clock in the evening, having been seen for three hours continuously."
Fig. 236.
Fig. 237.
348. Visual Angle.—In order that you may understand the operation of lenses in relation to vision I must first explain to you what is meant by the visual angle. In Fig. 236 (p. 270) are represented arrows of the same size at different distances from the eye. From the ends of each of the arrows are drawn lines to the eye. The angle which these lines make in each case as they meet at the eye is termed the visual angle. Now the apparent size of an object depends upon the size of this angle. The degrees of the angles are marked upon the figure. Thus the visual angle of the nearest arrow is 120 degrees, and that of the second is 60, only half as large. The first arrow therefore appears twice as large as the second. For the same reason it appears four times as large as the third, eight times as large as the fourth, and twelve times as large as the fifth. The same thing is illustrated in another way in Fig. 237. Here the arrows e f, g h, and i k appear to the eye as large as A B, because they have the same visual angle, and for this reason make an image of the same size in the eye, as you see is indicated in the figure. It is hardly necessary to say that what is true of objects as a whole is true also of any part of them. Each part, however small, has its visual angle, and this governs its apparent size.
Fig. 238.
