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This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1896 edition. Excerpt:...or of being opaque to all the planes but one. Cut two slices of this mineral in planes parallel with the long axis; when they are similarly placed an object can be seen through them both. But if one tourmaline be rotated, the object gets dimmer, and when the rotating slice is at right angles to the first, it disappears. At each quarter revolution it disappears and is restored. So it seems that light which has passed one transparent tourmaline differs so much from ordinary light that a second similar piece acts as an opaque body. The action may be illustrated by Fig. 97. The first set of vertical rods will allow all vertical planes Fig. 97. to pass, but stops those that are at right angles to these rods. Any plane that has passed one set will readily pass another similarly placed. But if grating B is turned so its rods are at right angles to the first, the plane that has passed A will be stopped by this second grating. Light having passed A is polarized, and the first piece of tourmaline is called the polarizer, and the second is the analyzer. To the eye there is no difference between common and polarized light. (2) Polarization by Reflection.--When a ray of light falls on a polished glass surface inclined to it at an angle of 35 25', it is reflected, and the reflected ray is polarized. To see that it is polarized, let the reflected ray be received on a second unsilvered surface, also inclined at the same angle and called the analyzer. This second surface will reflect the polarized ray if it is parallel to the first surface. If not parallel, no light is reflected; so these reflecting surfaces act much like the tourmaline crystals. What is the polarizing angle? The polarizing angle of a substance is the angle which the incident ray must make with...