Ordinary spar

For more than 100 years, researchers labored without being able to identify the elements in the stone. In 1774, Scheele announced the presence of a new element in the Bologna stone. Today, scientists know that the stone was a form of barite. Five years later, Scheele demonstrated that barite was also present in heavy spar. This dense transparent mineral closely resembles ordinary spar, a compound of calcium. 

The wave lengths of X-rays are too small to be measured in the same way as ordinary light waves. They have been measured by means of crystals. The atoms in a crystal like Iceland Spar or Diamond are arranged in regular layers at equal distances apart, as shown in Fig. 14. 

Occurs only in combination, principally as heavy spar (BaSO<)-and wifherite (BaCOs). It is a pale yellow, malleable metal, quickly oxidized in air, and decomposing HiO at ordinary temperatures. 

A rather curious observation was made in 1670 by Barthohnus, who discovered that when a beam of ordinary light passes through certain crystals, such as natural calcite or Icelandic spar, each ray splits into two. When he passed the two rays through a second crystal, he observed that the effect depends on the orientation... 

Iceland spar has the property of double refraction that is, it can split, or doubly refract, an entering beam of ordinary light into two separate emerging beams of light. Each of the two emerging beams (labeled A and B in Figure 23.2) has only a single plane of vibration, and the plane of vibration in beam A is perpendicular to the plane of beam B. In other words, the crystal has separated the incident beam of... 

Figure 6.26 The birefraction of natural light by a single crystal of spar CaCOji MN is the optical axis, o is the ordinary ray (o-ray), e is the extraordinary ray (e-ray).

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