of potash solution, the object of which is to free the transmitted sodium rays from any admixture of blue or green light. This is important in the case of solutions of high dispersive power, as, without it, other tints make their appearance when the Nicols are crossed, and 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 tube in Fig. 26 is one metre long; shorter tubes, of course, need cases of proportionate length. The case is filled with water, which is then raised to the desired temperature, usually 20° Cent., by moving about in it a hot bar, K. For higher temperatures a Bunsen lamp 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 to have the polarizer and analyzer properly placed at the ends. The supports A and D can be slid along and screwed to the cross-bars in other positions as required. In all exact observations it is necessary to determine the zeropoint afresh with each observation, as changes in the temperature of the place as well as differences of tension in the metallic screw-joints have an appreciable affect on the readings. The following values, obtained with a 10 per cent. solution of cane-sugar, are given as a working example:-The zero-points were found at about 20° and 200° on the right and left sides respectively 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 already come largely into use, affords results considerably more accordant than those obtained by the apparatus of Mitscherlich. Its novelty consists in the introduction of a Savart-prism 2 between the 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 flame is used as the source of light. 2 1 H. Wild: Ueber ein neues Polaristrobometer, Berne, 1865. [" The details of the instrument, as constructed by Hermann and Pfister, mechanicians, Berne,1 are shown in Figs. 27 and 28, the same parts being indicated in Fig. 27 by small and in Fig. 28 by large letters. A metal stand X, Fig. 28, supports a brass cradle Y, which is capable of vertical and horizontal movement, and carries at its extremities the polarizing and analyzing arrangements of the instrument. Entering at a, Fig. 27, into the dark chamber 6, the light passes through a circular diaphragm c (10 millimetres in diameter), thus reaching the Nicol d. The latter is fixed to the graduated disc e, with which it turns on a common axis. Thence, the polarized ray, after traversing the solution-tube f, passes on to the analyzer. Here it meets, first, the so-called Savart polariscope, a prism g, composed of two plates of calc-spar 3 millimetres thick, cut at an angle of 45° to their optic axes, and cemented together with their principal sections crossing each other at right angles. To this succeed two lenses forming a telescope of low power (about 5 times), the one, h, having a focal length of 120 millimetres, the other, i, of infinite length. Between the two, and in the focus of the objective h, is a circular diaphragm, k, of about 4 millimetres diameter, provided with cross-threads. Lastly comes the Nicol prism 1, fixed with its principal section horizontal. The latter will therefore form an angle of 45° with the principal sections of the double-plate g. That this relative position of the parts g and / may remain unaltered, the draw-tube, containing the Nicol and the lens i, is furnished with a guide-pin. The whole front part is set in the tube Z, which projects from the arm Y, and which allows it only a small movement about its own axis. For this purpose the tube is provided with a slot and adjusting screws mm, which clamp a projection on the inner tube. The object of the arrangement is for fixing the zero-point. Lastly, at n is placed a circular screen to shade the observer's eyes from extraneous light. The mode of effecting rotation of the polarizing Nicol is as follows:-The circular disc and the Nicol move in a piece within a fixed ring projecting from the arm Y. The disc is provided on the side next the observer with a toothed wheel driven by the pinion o, worked by the rod q, with 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 off 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, Stallschreiberstrasse 4, Berlin, also supply the instrument. piece t and objective u, and having at the farther end, v, an inclined metallic reflector with round hole in the centre, by means of which the light from a small gas-flame on a movable arm w, is thrown upon the vernier. In conclusion, it should be stated that the instrument is 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 |