108.  How Refraction Deceives us.  Refraction is the source of many illusions; bent rays of light make objects appear where they really are not.  A fish at A (Fig. 66) seems to be at B.  The end of the stick in Figure 64 seems to be nearer the surface of the water than it really is.

[Illustration:  FIG. 66.—­A fish at A seems to be at B.]

The light from the sun, moon, and stars can reach us only by passing through the atmosphere, but in Section 76, we learned that the atmosphere varies in density from level to level; hence all the light which travels through the atmosphere is constantly deviated from its original path, and before the light reaches the eye it has undergone many changes in direction.  Now we learned in Section 102, that the direction of the rays of light as they enter the eye determines the direction in which an object is seen; hence the sun, moon, and stars seem to be along the lines which enter the eye, although in reality they are not.

109.  Uses of Refraction.  If it were not for refraction, or the deviation of light in its passage from medium to medium, the wonders and beauties of the magic lantern and the camera would be unknown to us; sun, moon, and stars could not be made to yield up their distant secrets to us in photographs; the comfort and help of spectacles would be lacking, spectacles which have helped unfold to many the rare beauties of nature, such as a clear view of clouds and sunset, of humming bee and flying bird.  Books with their wealth of entertainment and information would be sealed to a large part of mankind, if glasses did not assist weak eyes.

By refraction the magnifying glass reveals objects hidden because of their minuteness, and enlarges for our careful contemplation objects otherwise barely visible.  The watchmaker, unassisted by the magnifying glass, could not detect the tiny grains of dust or sand which clog the delicate wheels of our watches.  The merchant, with his lens, examines the separate threads of woolen and silk fabrics to determine the strength and value of the material.  The physician, with his invaluable microscope, counts the number of infinitesimal corpuscles in the blood and bases his prescription on that count; he examines the sputum of a patient to determine whether tuberculosis wastes the system.  The bacteriologist with the same instrument scrutinizes the drinking water and learns whether the dangerous typhoid germs are present.  The future of medicine will depend somewhat upon the additional secrets which man is able to force from nature through the use of powerful lenses, because as lenses have, in the past, been the means of revealing disease germs, so in the future more powerful lenses may serve to bring to light germs yet unknown.  How refraction accomplishes these results will be explained in the following Sections.

110.  The Window Pane.  We have seen that light is bent when it passes from one medium to another of different density, and that objects viewed by refracted light do not appear in their proper positions.