It happens from this, that when in the uncovered half of the field, the plane of polarization has the direction AC, in the other half it will have the direction ACʹ. When the analyzer is rotated, if its plane of polarization lie in the direction cc, the rays polarized parallel to AC will be completely extinguished and the corresponding half of the field will be dark. The opposite happens when the plane of polarization lies in the direction of cʹcʹ. When one-half of the field is thus obscured, the other suffers only a partial diminution in the intensity of its illumination. When the middle position bb is reached in the rotation of the analyzer, the illumination of the two halves is uniform, and this is the point at which the zero of the scale is reached. The slightest rotation of the analyzer to the right or left of this neutral point will cause a shadow to appear on one of the halves of the field, which by an oscillatory movement of the analyzer, seems to leap from side to side. The smaller the angle _a_ or BAC, the more delicate will be the shading and the more accurate the observation. This angle being adjustable by the mechanism already described, should be made as small as will permit the admission of the quantity of light requisite for accurate observation. The various pieces composing the polariscope are arranged in the following positions, beginning on the right of Fig. 36, and passing to the left, where the observer is seated.[42] 1. The lamp VV, TT, AA, or the wheel burner: 2. The lens B for condensing the rays and rendering them parallel: 3. The tube I, blackened inside to carry the lens: 4. A thin lamina E, cut from a crystal of potassium bichromate, serving to render the sodium light more monochromatic: When the saccharine liquids under examination are colored the crystal of bichromate is removed before the observation is made. 5. The polarizer R, which is rotatable through a small angle by the lever K: 6. The lever JK for rotating the tube containing the polarizer: This is operated by the rod X extending to the left. 7. Diaphragm D, half covered with a lamina of quartz. 8. Trough L for holding the observation tube: In the large instrument shown in the figure, it is more than half a meter in length and arranged to hold an observation tube 500 millimeters long. 9. Disk C, carrying divided circle and arbitrary sugar scale: 10. Mirror M, to throw the light of the lamp on the vernier of the scale: [Illustration: FIGURE 36. LAURENT POLARISCOPE.] 11. Reading glass N, carried on the same radius as the mirror and used to magnify and read the scale: 12. Device F, to regulate the zero of the instrument: 13. Tube H, carrying a nicol analyzer and ocular O for defining the field of vision: This tube is rotated by the radial arm G, carrying the mirror and reading glass.