and this independently altogether of whether the proper position has been found by turning the analyzer to the right or to the left. It is to be observed, however, that when a Mitscherlich instrument is provided with any kind of Galilean telescope, the foregoing conditions are reversed. The position of the dark band indicates the point of extinction of the yellow rays.

2. When, on the other hand, the rotatory power of the active liquid is high, the dark band appears broad and undefined, or else cannot be brought back by any movement of the analyzer at all. By turning the latter we get merely a succession of colours, produced by the analyzer extinguishing, according to the position of its principal section, certain of the unequally-rotated coloured rays, and allowing the rest to pass on with different intensities, thus producing a succession of colour-mixtures. With solutions in which the angle of rotation for any ray is less than 90°, the sequence of coloured tints, when the analyzer is turned from the initial zero-point, is as follows:

As a point of reference for the analyzer, that position is chosen where the transition from blue to red stands exactly in the middle of the field of vision. This being the point of extinction of the yellow rays, the observed angles will be a.

With substances of still higher rotatory and dispersive powers, intermediate tints make their appearance; for example, between the blue and the red a reddish-violet, which, on the least touch of the analyzer, passes into one or the other. This is known as the sensitive or transition tint, and appears when the position of the analyzing Nicol is exactly such as to bar the passage of the mean yellow rays. This position also gives the angle a.

For the relation between the angles of rotation a; and a, see § 18. Observations taken with white light cannot be made so exactly as those with the sodium flame; the former light is therefore only employed when the latter is not available.

$48. Mitscherlich's larger Instrument for Observations at Constant Temperature.-In exact researches it is requisite, as already stated (§ 22), that the temperature should not only be known, but be constant during the period of observation. This can only be effected by surrounding the tube with water. Moreover, in examining substances of feeble rotatory power, it is necessary to employ tubes of considerable length, sometimes a whole metre long, to obtain rotation-angles of sufficient magnitude. Fig. 26 represents an instrument fulfilling these conditions, constructed at the works of Dr. Meyerstein, of Fig. 26.

Göttingen, and in use in the chemical laboratory of the Polytechnic School at Aachen.

Starting from the end next the light, the instrument consists of the following parts, resting loosely upon a frame formed of two strong iron bars QQ:

1. A fixed tube A, containing the polarizing Nicol, a convex lens of long focus, and a diaphragm with a square aperture of 5 millimetres side. Affixed to the same support as the tube is a circular dark screen, a, to shut off extraneous light.

2. A glass bottle with parallel walls B,1 filled with bichromate

1 May be obtained of Dr. J. G. Hofmann, 29, Rue Bertrand, Paris.

f potash solution, the object of which is to free the transmitted odium rays from any admixture of blue or green light. This is mportant in the case of solutions of high dispersive power, as, withut it, other tints make their appearance when the Nicols are crossed, nd interfere with the sharp recognition of the dark band.

3. A sheet-metal case, C C, through which the solution tube passes, the ends passing water-tight through india-rubber corks. The ube in Fig. 26 is one metre long; shorter tubes, of course, need ases of proportionate length. The case is filled with water, which s then raised to the desired temperature, usually 20° Cent., by noving about in it a hot bar, K. For higher temperatures a Bunsen amp with a row of burners, JJ, must be used.

4. A support, D, carrying a tube containing the analyzing Nicol, which, together with the graduated disc attached to it, is susceptible of movement round a common axis. This movement is communicated by the screw G, working in the toothed rim of the disc E. A small Galilean telescope, F, is fitted to the tube, the eye-piece of which must be so adjusted that the aperture in the diaphragm of the polarizer appears sharply defined. The support also carries two fixed verniers, and the divisions can be read off by the light of a small gas-jet, H.

The sodium flame for the observations is obtained by means of the blow-pipe L, arranged vertically with chimney, M, over it, and connected by means of india-rubber tubing with the bellows P. Over the nozzle of the burner, and projecting from the support N, is fixed a ring of platinum wire, which, when dipped in fused soda, imparts to the whole mass of flame an intense yellow, thus producing a strong light, essential for observations with great lengths of liquid, since the slightest opacity will, in such cases, often obscure the field so much as entirely to frustrate the experiment.

The rotation-angles are determined in the same way as with Mitscherlich's smaller instrument. The analyzer is turned, by means of the milled head G, until the dark band appears exactly in the middle of the field of vision formed by the square aperture of the diaphragm, or, in other words, until the light spaces on each side of the dark band appear of equal width. The tube is first introduced empty, and the two zero-points determined, after which it is filled with the active liquid, the metal case being turned up on one end for the purpose. When shorter tubes are used, the intervention of extraneous light must be prevented by enclosing the course of the rays after they

leave the tube with a paste-board cylinder, always taking care t have the polarizer and analyzer properly placed at the ends. Th supports A and D can be slid along and screwed to the cross-bar in other positions as required.

In all exact observations it is necessary to determine the zero point afresh with each observation, as changes in the temperature o the place as well as differences of tension in the metallic screw-joint have an appreciable affect on the readings.

The following values, obtained with a 10 per cent. solution o cane-sugar, are given as a working example:-The zero-points wer found at about 20° and 200° on the right and left sides respectivel of 0° and 180°. The vernier could be read accurately to 0.1° and approximately to 0.01°. The temperature of the solution was 20° Cent

(b.) Wild's Polariscope.

§ 49. The polariscope invented by Wild1 in 1864, which has ready come largely into use, affords results considerably more cordant than those obtained by the apparatus of Mitscherlich.

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Its novelty consists in the introduction of a Savart-prism between he polarizer and analyzer (the former of which has the rotatory movement), whereby a number of parallel interference-bands are brought into the field of vision, which vanish in certain positions of the polarizer. These positions, which can be determined with great accuracy, furnish the reference marks of the instrument. A sodium fame is used as the source of light.

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