The details of the instrument, as constructed by Hermann ar Pfister, mechanicians, Berne,1 are shown in Figs. 27 and 28, tl same parts being indicated in Fig. 27 by small and in Fig. 28 large letters. A metal stand X, Fig. 28, supports a brass cradle Y, which capable of vertical and horizontal movement, and carries at its e tremities the polarizing and analyzing arrangements of the instrumen Entering at a, Fig. 27, into the dark chamber 6, the light pass through a circular diaphragm e (10 millimetres in diameter), th reaching the Nicol d. The latter is fixed to the graduated disc with which it turns on a common axis. Thence, the polarized ra after traversing the solution-tube ƒ, passes on to the analyzer. He it meets, first, the so-called Savart polariscope, a prism g, composed two plates of calc-spar 3 millimetres thick, cut at an angle of 45° their optic axes, and cemented together with their principal section crossing each other at right angles. To this succeed two lenses formir a telescope of low power (about 5 times), the one, h, having a foc length of 120 millimetres, the other, i, of infinite length. Between tl two, and in the focus of the objective h, is a circular diaphragm, k, about 4 millimetres diameter, provided with cross-threads. Last] comes the Nicol prism 7, fixed with its principal section horizonta The latter will therefore form an angle of 45° with the principal se tions of the double-plate g. That this relative position of the parts and 7 may remain unaltered, the draw-tube, containing the Nicol an the lens i, is furnished with a guide-pin. The whole front part is set i the tube Z, which projects from the arm Y, and which allows it onl a small movement about its own axis. For this purpose the tube provided with a slot and adjusting screws mm, which clamp a pro jection on the inner tube. The object of the arrangement is fo fixing the zero-point. Lastly, at n is placed a circular screen to shad the observer's eyes from extraneous light. The mode of effectin rotation of the polarizing Nicol is as follows:-The circular disc an the Nicol move in a piece within a fixed ring projecting from th arm Y. The disc is provided on the side next the observer with toothed wheel driven by the pinion o, worked by the rod q, wit milled-head p. The graduation is close to the edge of the disc, and in front of it is a fixed vernier or simply an index arm r. To read of the divisions a telescope, s, is used, consisting of the movable eye 1 Dr. J. G. Hofmann, 29, Rue Bertrand, Paris, and Schmidt and Haensch, Stall schreiberstrasse 4, Berlin, also supply the instrument. ece t and objective u, and having at the farther end, e, an inclined. etallic reflector with round hole in the centre, by means of which e light from a small gas-flame on a movable arm w, is thrown upon e vernier. In conclusion, it should be stated that the instrument generally constructed for tubes 220 millimetres (8.6 in.) long.1 1 Should the instrument be wanted for general scientific work and not merely as a saccharimeter, it is necessary to take care that the disc be graduated all round, and not § 50. That the liquids may possess a fixed temperature maintained constant, the experimental tube requires to be laid in a water-bath For this purpose it is enclosed in a metal jacket of considerably larger diameter, so as to allow a current of water to flow between (see § 65, Fig. 44). The complete arrangement of the apparatu ready for observation, is shown in Fig. 29. The instrument is set up opposite a Bunsen lamp B, which, with its bead of common salt, gives the sodium flame (or the lamp show in § 22, Fig. 46, may be used instead). The outer case of the tube i provided with two side pieces, of which the lowermost is connected by india-rubber tubing, D, with the reservoir 4, while the other at serves as an outflow-pipe. A third opening, E, is for the insertion of a thermometer in the water as it flows through. The zinc reservoir A, resting upon a tall iron stand, is provided with a stirrer F, and is swathed in flannel to prevent loss of heat. One or two thermometers, G, serve to indicate the temperature of the water, usually maintained at 20°, by means of the lamp H. The water is allowed to flow through the tube for about twenty minutes before the observations are begun, the stream being interrupted during observation, while the thermometer E must remain steady throughout. The instrument should be set up in a darkened room; the gas-jet required for Fig. 30. reading the scale is controlled by the stop-cock I. At K is represented a short (100 millimetre) tube, provided with a junction-piece to allow of its insertion in the instrument. § 51. In carrying out observations a tube, empty at first-in order to fix the zero-points—is introduced and the eye-piece drawn out, so that the cross-threads appear sharply defined. The polarizing Nicol must then be turned by means of the milled-head, p. 107, Fig. 27, until a number of parallel dark bands or fringes make their appearance in the field of vision (Fig. 30, a). As the move in one or two quadrants only. The instruments made by Hermann and Pfister have their discs divided to the third of a degree (20 minutes), and either a vernier to give readings to 5 minutes, or else a simple index-point which suffices to read, to the same amount approximately. The polariscopes made by Dr. Hofmann, of Paris, read to single minutes. It would be more convenient if the divisions on the scales were not minutes but decimals of a degree (as 0·02°), as it is always in this form that the rotation angle of active substances is expressed. In reading as minutes one has to convert into decimals of a degree by dividing by 0.6. ent continues these become fainter, until at last a position is reached which a luminous space, devoid of lines, occupies the field. By a ight movement of the milled-head to and fro this luminous space brought, as nearly as possible, into the middle of the field of ision, so that the remains of the fringes appear to stand at equal istances to the right and left of the cross-threads (see Fig. 30, b). 'his position serves as reference-point for the angular measurement. If the Nicol be turned further, the dark lines will grow darker till hey attain a certain maximum intensity, then become fainter again, nd again vanish; these maxima recurring at intervals of 90° in the ourse of a complete revolution.1 Generally the dark lines exhibit ertain peculiarities of form in each position which can be recognized.2 Their disappearance indicates positions of the movable Nicol, in which its principal section either coincides with, or is perpendicular co the plane of the principal section of the first of the calc-spar plates of the Savart, while they occur with maximum intensity when these 1 For the theory of these interference-bands, see Wild, Polaristrobometer, Berne, 1865; or Wüllner, Lehrbuch der Physik, 3 Aufl. Bd. 2, 604. 2 In many Wild's polariscopes the luminous space is too wide to allow any remains of the dark lines to show on the right and left of it. Some other reference position must then be chosen. Perhaps the best method is when the dark lines have nearly passed out of the field to fix the eye on one side of the field of vision, say, the right, and continue turning the analyzer slowly until the last traces of the lines disappear at that side. This position will necessarily be the same in every observation-that is to say, the milled-head p will always have communicated to the polarizer precisely the same amount of adjustment in each case. The cross-threads are not required here. With many people, however, this method does not afford the same amount of accuracy as by bringing the fringes into a position at equal distances from the cross-threads on each side. In the case of an instrument in which the luminous space is too wide, Tollens (Ber. der deutsch. chem. Gesell. 10, 1405) adopts the plan of loosening the adjustment-screws and turning the ocular draw-tube, containing the stationary Nicol 1, Fig. 27, through an angle of 20° to 40° on its own axis. In this way the phenomena presented by a complete revolution of the analyzer are altered, the field becoming more or less darkened at two points 180° apart, and acquiring at two intermediate points a maximum luminosity. The dark lines vanish at each position as before, but while in the illumined quadrants the luminous space is now broader, in the darkened quadrants it is narrower than before. The former positions are entirely unsuitable for purposes of observation; but the two latter permit of a very sharp adjustment of the fringes in regard to the cross-threads. The observations must then be made in these two quadrants only. See, also, Tollens (Ber. der deutsch. chem. Gesell. 11, 1804). Some instruments are so constructed that the interference-bands appear vertical, in which case the cross-threads are placed horizontally. In Wild's instruments, should other lines appear crossing the field of vision obliquely, it is a sign that the principal sections of the two calc-spar plates of the Savart are not truly perpendicular to each other, and the instrument should be returned to the maker for readjustment two planes form angles of 45°. The parts are generally so regulated by the maker that the position of the index at the absorption-points is approximately at the readings 0°, 90°, 180°, 270° on the scale, admitting, however, of a certain amount of adjustment by means of the screws m m, Fig. 27. If the Nicol is set to one of the four zero-points, and the empty tube replaced by one filled with some optically-active liquid, the interference-bands reappear. In its passage through the active medium the plane of polarization of the transmitted ray is made to rotate through a certain angle, and to restore it to a position either parallel or perpendicular to the principal section of the first calcspar plate of the Savart, the Nicol must be turned in the opposite direction to that in which the rotation has taken place, when the fringes will again disappear. The graduated disc must therefore be turned to the left if the substance is dextro-rotatory, and to the right if lævorotatory. But, as regards the movement of the milled-head Pp, Figs. 28 and 27, inasmuch as a change of direction is involved in the wheel-and-pinion movement, the direction in which the milled-head is turned must be the same as that of the rotation. When, as usually happens, the graduation follows the same direction as the figures on a watch-dial, the readings for a dextro-rotatory substance will be greater, and for a lævo-rotatory substance less than the number on the disc at the zero-point. § 52. If the direction of the rotatory power of an active liquid be unknown, it will be best to begin observations with a weak solution in the tube, so as to get a feeble amount of rotation. It will then be easy to see whether the direction is to the right or the left of the zero-point. On the contrary, when the rotation is considerable, a doubt may remain as to the direction, in the same way as with Mitscherlich's instrument (§ 46). For instance, let the four zero-points be 0° 90° 180° 270° and after the insertion of the tube with its liquid, the vanishing points of the dark lines, 30° 120° 210° 300°. Here the medium may be, as shown in Figs. 31 and 32, either dextrorotatory with an angle of 30°, or lævo-rotatory with an angle of 60°. To decide the question, a second observation is necessary with a shorter tube, or a more dilute solution. If the length of tube or |