tion 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.

When a ray of light passes through a piece of plane glass, such as a
window pane (Fig. 67), it is refracted at the point _B_ toward the
perpendicular, and continues its course through the glass in the new
direction _BC_. On emerging from the glass, the light is refracted
away from the perpendicular and takes the direction _CD_, which is
clearly parallel to its original direction. Hence, when we view
objects through the window, we see them slightly displaced i