Page:The American Cyclopædia (1879) Volume XV.djvu/459

 SUGAR 443 mon lamp is passed through the aperture S and the double-refracting prism r, the polari- zer which transmits the ordinary ray, the ex- traordinary being thrown out of the field of vision. (See LIGHT, vol. x., pp. 445, 446.) The prism is so placed that the plane of polari- Soleil's Saccharimeter. zation is in the axis of the instrument and also vertical. After passing through the double- refracting prism the polarized ray meets a re- fracting plate q, shown in section at E, com- posed of two pieces of quartz placed side by side, one having right-handed and the other left-handed polarizing powers. These plates are each 3'Y5 millimetres thick, producing a rotation of 90 and a rose-violet tint, called the transition tmt. These two quartz plates, having equal powers of rotation, turn the ray in opposite directions, and therefore when viewed through a double-refracting prism they appear of the same tint when the plane of the ray is perpendicular ; but if it has been turned by passing through a rotating solution, a dif- ference of tint will be produced. After pass- ing through the double quartz plate #, the ray traverses the solution in the tube m, and a sin- gle quartz plate i, fig. B, of any thickness and either right-handed or left-handed. The com- pensator w, composed of two wedge-shaped pieces of quartz, shown in section at C, both either right-handed or left-handed, but of op- posite rotation to the plate ', is next traversed by the ray. This compensator can be varied in thickness and therefore in rotating power so as to balance exactly the degree of rotation produced by the solution. Its thickness is regulated by means of a rackwork and pinion turned by the milled head screw 5, figs. A and B. A scale and vernier shown at D is affixed to the plates, by which the thickness of the compensation may be read, the vernier point- ing to zero when the thickness of the two plates is equal to that of i. A double-refracting prism c, fig. B, is placed next behind the com- pensator to act as an analyzer which has been acted upon by the solution and the various plates. When the liquid in the tube is inac- tive and the compensator is not at zero, the plate i and the compensator will neutralize each other's effect, and the two parts of the double quartz q will have the same tint ; but when the tube m contains a solution having a rotatory power, like sugar, this power added to that of one of the plates will rotate the plane of polarization of the transmitted ray either to the right or to the left. If the solution con- tains cane sugar or dextrose, or a certain excess of either, it will rotate it to the right ; if it con- tains Ia3vulose or a certain excess, it will rotate it to the left, and therefore a difference in tint will be observed in the two halves of the double quartz plate -, one half perhaps being red and the other blue. The thickness of the compen- sator is then adjusted by turning the milled head 5 until the tints become the same, and the increase or decrease in the thickness of the two plates will indicate the rotatory power of the solution, either right-handed or left-handed, and may be read upon the scale. The following standard of comparison is employed : If 16-471 grs. of pure cane sugar is dissolved in sufficient water to make 100 cubic centimetres, this solu- tion placed in a tube 20 centimetres long will produce the same degree of rotation as a right- handed quartz plate one millimetre thick. Or if a tube exactly 37'65 in. long is filled with a solution containing 10 per cent, of pure cane sugar (crystallized sugar candy), and a polar- ized ray from the middle of the yellow band of the spectrum is passed through it, the ro- tation of th<j plane will be V3'8. This, com- pared to the rotation produced by a solution of pure cane sugar of a different strength, will show the relative proportion it contains; or if the depth of the solution is less, the rota- tion will be less in the same proportion. If the solution contains left-handed sugar, the result will be vitiated and corrections have to be made. This may be done by converting all the sugar into left-handed sugar by the action of hydrochloric acid, and making a second ob- servation, when by a comparison of the results obtained at both observations the amount of cane sugar may be estimated. The optical ro- tatory power of the various sugars mentioned in this article has been determined^ according to the standard of comparison here given. The arrangement of prisms and lenses placed behind the double-refracting prism c forms what M. Soleil calls the producer of sensible tints. ^ The particular tint which allows the most delicate difference in the color of the two halves of the double quartz to be distinguished is not the same for all eyes. This effect is produced by placing in front of the prism c a quartz plate o cut perpendicular to the axis, then a small Galilean telescope, consisting of a double con- vex lens g and a double concave lens f, with adjustable focal distance. The double-refract- ing prism c acts as polarizer to the quartz o, while the prism a is the analyzer, and on being