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various angles and elevations, and determine the position in which the most brilliant effects are produced. The same results are ob
tained if the eye is placed below the glasses in such a position that the polarized ray is reflected from B.
554. Double Refraction. If a crystal of Iceland spar AB (Fig. 521) is placed upon a sheet of paper on which there is a black dot, the eye placed above the spar will see two dots, one a short distance from the other and apparently above the surface of the paper. This separation of the light into two rays shows that light passes through the crystal more rapidly in one direction than in the other and is called double refraction.
Two rays from each point reach the eye by different paths: one of them obeys the ordinary laws of refraction, and is called the ordinary ray; the other does not obey these laws, and is called the extraordinary ray. Another peculiarity of these rays is that they are polarized.
If a crystal of Iceland spar is cut along the diagonal AB and then cemented together again with Canada balsam, it has the property of permitting only the extraordinary ray to emerge. This arrangement is called a Nicol's prism, and is much used in the study of polarized light.
555. The Polariscope. The study of substances by polarized light has become a matter of great importance on account
of its use in the detection of adulteration; the substitution of grape sugar for cane sugar, for example. A simple polariscope (Fig. 522) serves to demonstrate many of the phenomena of polarized light nearly as well as more elaborate apparatus.
FIG. 522.- Polariscope
The base AB supports a vertical ground-glass plate C. A piece of black glass, or a glass plate over a sheet of black paper, is laid on the base at D. An arm E is fixed to the base at an angle of about 35°. K supports a Nicol's prism and L the object to be studied. Mica cut in different thicknesses gives beautiful color effects. The strained condition of the glass in a pressed bottle stopper can be detected by the appearance of dark spots as the Nicol's prism is rotated.
1. Make a drawing of a prism, the refractive angle A of which is 60°, and trace the path of a ray from L as it passes through and out of the prism. Using the same incident ray L, increase the angle A by 10° successively, and show the change in the emerging ray until it no longer emerges from the side AC. Where does the ray emerge?
2. What shape of rainbow might it be possible to see from a balloon?
3. If, in examining with a spectroscope the light from a distant body,
you should find the spectrum continuous, what conclusion would you draw?
4. The velocity of light has been given as 300,000 km. per second. Take the wave length of the red (B) line from the table in §547, and find how many waves of that color strike the eye per second.
5. In observing the spectrum of a star, the D line was found to be displaced toward the violet end of the spectrum. What is the meaning of the displacement?
6. In pressing two pieces of glass together in a photographic printing frame, a number of groups of colored lines sometimes appear. What is the explanation?
7. The light used in a photographic dark room is a red light. Why?
V. OPTICAL INSTRUMENTS
556. The Simple Microscope is merely a convex lens, usually of short focal length. The object is placed between the principal focus and the lens, and the image is virtual, upright, and larger than the object. If the object AB (Fig. 524) is placed as shown, the position of the image of the point A is determined by the intersection, at A', of aF' and 40. B' is found in a similar way, and the positions of these two points determine the position of the whole image. The distance from the eye to the image is the distance of distinct vision and varies with different eyes.
557. The Compound Microscope (Figs. 525 and 526). The simplest form of the compound microscope consists of two converging lenses: an object glass O and an eyepiece E.
The distance between these is so arranged that the object glass forms a real, enlarged, inverted image of the object between the eyepiece and its focus. The function of the
eyepiece is to enlarge this image, so that the eye sees the enlarged and inverted image at ab.
558. The Astronomical Telescope. -The ordinary astronomical telescope (refracting) is much like the compound microscope in principle. But while both lenses magnify in the microscope, owing to the nearness of the object to the object glass, the eyepiece alone magnifies in the telescope. This is due to the fact that the object is at a distance, and the image formed by the object glass is nearly at its principal focus and very small.
559. The Terrestrial Telescope. The image given by the astronomical telescope is an inverted one. The inverting of the image is not very objectionable when one is looking