If a ray of common light be made to pass through certain crystals, such as calc spar, it undergoes double refraction, and the light transmitted becomes polarised. The arrangement known as Nicol’s prism, which consists of two prisms of calc spar, cut at a certain angle and united together by means of Canada balsam, is a very convenient means of obtaining polarised light. If two Nicol’s prisms are placed in a similar position, one behind the other, the light polarised by the first (or polarising) prism passes through the second (or analysing) prism unchanged; but if the second prism be turned until it crosses the first at a right angle, perfect darkness ensues. While it would exceed the limits of this work to enter fully upon the theoretical explanations which are commonly advanced concerning the cause and nature of this polarised, or transformed light, it may be well to state here that common light is assumed to be composed of two systems of beams which vibrate in planes at right angles to each other, whereas polarised light is regarded as consisting of beams vibrating in a single plane only. If, now, we imagine the second Nicol’s prism to be made up of a series of fibres or lines, running only in one direction, these fibres would act like a grating and give free passage to a surface like a knife blade only when this is parallel to the bars, but would obstruct it if presented transversely. This somewhat crude illustration will, perhaps, serve to explain why the rays of light which have been polarised by the first Nicol’s prism are allowed to pass through the second prism when the two are placed in a similar position, and why they are obstructed when the prisms are crossed at right angles, it being remembered that in a polarised ray the vibrations of the beams of light take place in a single plane.
Suppose we place between the two Nicol’s prisms, while they are at right angles, a plate cut in a peculiar manner from a crystal of quartz, we will discover that rays of light now pass through the second prism, and that the field of vision has become illuminated with beautiful colours—red, yellow, green, blue, etc., according to the thickness of the quartz plate used. On turning the second Nicol’s prism on its axis, these colours will change and pass through the regular prismatic series, from red to violet, or the contrary, according to the direction of the rotation produced by the intervening plate. Quartz, therefore, possesses the remarkable property of rotating the plane of polarisation of the coloured rays of which light is composed; and it has been discovered that some plates of this mineral exert this power to the right, others to the left; that is, they possess a right or left-handed circular polarisation. Numerous other substances, including many organic compounds, possess this quality of causing a rotation—either to the right or left—of a plane of polarised light. For example, solutions of cane sugar and ordinary glucose cause a right-handed rotation, whilst levulose and invert sugar exert a left-handed rotation. The extent of this power is directly proportional to the concentration of the solutions used, the length of the column through which the ray of polarised light passes being the same. It follows that on passing polarised light through tubes of the same length which are filled with solutions containing different quantities of impure cane sugar, an estimation of the amount of pure cane sugar contained in the tubes can be made by determining the degree of right-handed rotation produced; and it is upon this fact that the application of the polariscope in sugar analysis is based. The optical portions of the most improved form of the polariscope—that known as the Ventzke-Scheibler—are represented by Fig. 2.
The light from a gas burner enters at the extremity of the instrument and first passes through the “regulator A,” which consists of the double refracting Nicol’s prism a and the quartz plate b, it being so arranged that it can be turned round its own plane, thus varying the tint of the light used, so as to best neutralise that possessed by the sugar solution to be examined. The incident ray now penetrates the polarising Nicol’s prism B, and next meets a double quartz plate C (3·75 millimetres in thickness). This quartz plate, a front view of which is also shown in the figure, is divided in the field of vision, one half consisting of quartz rotating to the right hand, the other half of the variety which rotates to the left hand. It is made of the thickness referred to owing to the fact that it then imparts a very sensitive tint (purple) to polarised light, and one that passes very suddenly into red or blue when the rotation of the ray is changed. Since the plate C is composed of halves which exert opposite rotary powers, these will assume different colours upon altering the rotation of the ray. After leaving the double quartz plate the light, which, owing to its passage through the Nicol’s prism B is now polarised, enters the tube D containing the solution of cane sugar under examination; this causes it to undergo a right-handed rotation. It next meets the “compensator” E, consisting of a quartz plate c, which has a right-handed rotary power, and the two quartz prisms d, both of which are cut in a wedge shape and exert a left-handed rotation. They are so arranged that one is movable and can be made to slide along the other, which is fixed, thus causing an increase or decrease in their combined thickness and rotary effect. The ray of light then passes through the analysing Nicol’s prism F, and is finally examined by means of the telescope G, with the objective e and ocular f. Fig. 3 gives a perspective view of the Ventzke-Scheibler polariscope. The Nicol’s prism and quartz plate which constitute the “regulator” are situated at A and B, and can be rotated by means of a pinion connecting with the button L. The polarising Nicol’s prism is placed at C, and the double quartz plate at D. The receptacle h contains the tube P filled with sugar solution, and is provided with the hinged cover h´, which serves to keep out the external light while an observation is being taken. The right-handed quartz plate and the wedge-shaped quartz prisms (corresponding to c and d, Fig. 2) are situated at G, and at E and F, and the analysing Nicol’s prism is placed at H. When the wedge-shaped prisms have an equal thickness coinciding with that of the quartz plate c (Fig. 2) the left-handed rotary power of the former is exactly neutralised by the right-handed rotary power of the latter, and the field of vision seen at I is uniform in colour, the opposing rotary powers of the two halves of the double quartz plates C (Fig. 2) being also equalised. But if the tube, filled with a sugar solution, is placed in the instrument, the right-handed rotary power of this substance is added to that half of the double quartz plate which exerts the same rotary effect (the other half being diminished in a like degree), and the two divisions of the plate will now appear of different colours. In order to restore an equilibrium of colour the movable wedge-shaped quartz plate E is slid along its fellow F by means of the ratchet M, until the right-handed rotary power of the sugar solution is compensated for by the increased thickness of the left-handed plate, when the sections of the plate C will again appear uniform in colour. For the purpose of measuring the extent to which the unfixed plate has been moved, a small ivory scale is attached to this plate, and passes along an index scale connected with the fixed plate. The degrees marked on the scale, which are divided into tenths, are read by aid of a mirror s attached to a magnifying glass K. When the polariscope is in what may be termed a state of equilibrium, i. e. before the tube containing the sugar solution has been placed in it, the index of the fixed scale points to the zero of the movable scale.
In the practical use of the Ventzke-Scheibler saccharimeter the method to be followed is essentially as follows: 26·048 grammes of the sugar to be tested are carefully weighed out and introduced into a flask 100 cubic centimetres in capacity; water is added, and the flask shaken until all crystals are dissolved. The solution is next decolorised by means of basic plumbic acetate, its volume made up to 100 cubic centimetres, and a little bone-black having been added if necessary, a glass tube, corresponding to P (Fig. 3) which is exactly 200 millimetres in length, and is provided with suitable caps, is completely filled with the clear filtered liquid. This is then placed in the polariscope, and protected from external light by closing the cover shown at h´. On now observing the field of vision by means of the telescope, it will be seen that the halves into which it is divided exhibit different colours. The screw M is then turned to the right until this is no longer the case, and absolute uniformity of colour is restored to the divisions of the double quartz plate C (Fig. 2). The extent to which the screw has been turned, which corresponds to the right-handed rotation caused by the sugar solution, is now ascertained on reading the scale by the aid of the glass K. The instrument under consideration is so constructed that, when solutions and tubes of the concentration and length referred to above are used, the reading on the scale gives directly the percentage of pure crystallisable cane sugar contained in the sample examined. For instance, if the zero index of the fixed scale points to 96°·5 on the movable scale, after uniformity of colour has been obtained, the sample of sugar taken contains 96·5 per cent. of pure cane sugar. The results given by the polariscope possess an accuracy rarely, if ever, attained by any other apparatus employed in the determination of practical commercial values.[57]
The proportion of grape sugar intentionally added to cane sugar can also be determined by the use of the polariscope, certain modifications being observed in its application. As previously stated, cane sugar is converted into a mixture of dextrose and levulose, termed invert sugar, by the action of dilute acids. While the rotary effect of dextrose upon the plane of a ray of polarised light is constant at temperatures under 100°, that exerted by levulose varies, it being reduced as the temperature is increased; hence it follows that at a certain temperature the diminished levo-rotary power of the levulose will become neutralised by the dextro-rotary effect of the dextrose, i.e. the invert sugar will be optically inactive. This temperature has been found to approximate 90°. Since dextrose is not perceptibly affected by the action of weak acids, it is evident that by converting cane sugar into invert sugar and examining the product by the polariscope at a temperature of about 90°, the presence of any added dextrose (glucose) will be directly revealed by its dextro-rotary action. This is accomplished by a method suggested by Messrs. Chandler and Ricketts,[58] which consists in substituting for the ordinary observation tube of the polariscope a platinum tube, provided with a thermometer, and surrounded by a water-bath, which is heated to the desired temperature by a gas burner (Plate X. Fig. 4). The sugar solution to be examined is first treated with a little dilute sulphuric acid, then neutralised with sodium carbonate, clarified by means of basic plumbic acetate, filtered, and the polariscopic reading taken at a temperature of 86° to 90°.
Since the results given by the foregoing method represent pure dextrose, it is necessary to first ascertain the dextro-rotary power of the particular variety of glucose probably employed for the adulteration of the sugar under examination, and then make the requisite correction. This process for the estimation of glucose is especially advantageous, in that the optical effect of the invert sugar normally present in raw cane sugars is rendered inactive.
It is sometimes desirable to determine the relative proportions of the organic constituents which are present in commercial glucose. These usually consist of dextrose, maltose, and dextrine, all of which possess dextro-rotary power, but not in the same degree; that of dextrose being 52, that of maltose 139, and that of dextrine 193. An estimation of the amount of each can be made by first ascertaining the total rotary effect of the sample by means of the polariscope.[59] This is expressed by the equation
P = 52 d + 139 m + 193 d′,
(1)