beams that pass through the tubes containing the solution and solvent. The beams are then directed by rhombs and lenses into the Martenstype photometer.

For reflectance measurements (lower part of diagram) the first pair of rhombs is removed, the two beams coming directly from the surfaces of sample and reference standard. The sample and standard are carried on a holder and may be interchanged in vertical position. This position is not suitable for dry, granular sugars.

The photometric field is circular with horizontal dividing line, being projected through the entrance slit to a position in the collimator objective where it is viewed from the ocular slit without eye lens. Transmission and reflectance are computed by the formulas used with the instrument described under (a), p. 307. The instrument also

FIGURE 62.-Gaertner polarizing spectrophotometer.

may be made direct reading if the angle of match (100 point) can be secured at exactly 45°.

(c) KÖNIGS-MARTENS SPECTROPHOTOMETER WITH AUXILIARY EQUIPMENT DESIGNED AT THE NATIONAL BUREAU OF STANDARDS

The following description of this apparatus is a part of that given by Gibson [9], and figure 63 is a reproduction of figure 3 of an article by McNicholas [10].

Light entering the collimator slits, A and B, forms beams 1 and 2. These beams follow the usual course through the collimator lens, dispersing prism, and telescope objective. Cemented to the latter lens is a combination of Wollaston prism, wedge, and biprism. The purpose of the wedges in the collimator and telescope is to prevent passage to the eye of certain multiply reflected rays from the optical surfaces.

Looking through the ocular slit one sees the surface of the biprism uniformly illuminated by a mixture of light of wave-length range determined by the widths of the collimator and ocular slits, the mean wave length corresponding to the position of the ocular slit in the spectrum. The biprism edge forms a vertical dividing line in this photometric field which is circular and the lights in the two halves of the field are plane-polarized in directions perpendicular to each other.

By turning the nicol between the eye and the ocular slit, the two parts of the field may be matched in brightness.

For spectral-transmission measurements three different illuminants are used, viz, the mercury arc, the helium lamp, and the incandescent lamp. Only the latter is shown in the diagram. Each of the three illuminants is mounted in a small enclosure, the inside surface of which is coated with MgO. In each case the light used for the transmission measurements is taken from the diffusing rear surface of the enclosure; this is collimated by the combination lenses (3) and enters the collimator slits, A and B, as shown.

The drum in which the analyzing nicol is mounted is graduated only in degrees reading from 0° to 360°. Since the angle of match does not fall exactly at 45°, the scale is not direct reading. The method of interchanging the sample in the two beams is used, and T is computed by formula:

T cot 01Xtan 02.

The mercury and helium illuminants afford ready means of checking the wavelength calibration of the spectrophotometer; they also enable measurements of

transmission to be made at certain wave lengths free from slit-width or wavelength error.

The rotating sector, shown in the diagram between the collimator slit and the transmission sample, serves two purposes: (1) It enables a direct check to be made on the reliability of the photometric scale. A number of sectors are available to give transmissions of approximately 0.01 to 0.80. These apertures are accurately determined mechanically. Any desired sector may be placed in position, rotated rapidly enough to eliminate flicker, and its transmission determined photometrically in the same manner as for the transmission sample. (2) To measure low transmissions, the 0.10 or 0.01 sector is placed in the blank beam and the transmission of the sample is measured relative to that of the sector. This brings the angles of match away from the extinction points into a more suitable region of the scale and greatly extends the range of the instrument for low transmission measurements.

For spectral reflectance measurements, the sample and reference white standard are placed, as shown, at the base of a hemisphere whose interior surface is coated with MgO and studded with 156 small lamps so that, in effect, the sample and standard are in completely diffused illumination. The light reflected at right angles from the sample and standard, respectively, forms beams 1 and 2 and enters the spectrophotometer via right-angled prism 4 and lenses 1 and 2. Sample and standard may be reversed in position by the observer and the apparent

reflectance of the sample, R., relative to that of the standard, R。, is given by the relation, analogous to formula 89,

(d) KEUFFEL & ESSER COLOR ANALYZER

This spectrophotometer is designed for both transmission and reflectance measurements and the arrangement of parts and the light paths are shown in figure 64, as given in the article by Keuffel [13]. The white-lined sphere is illuminated by two 400-watt lamps. For transmission measurements, two blocks of magnesium carbonate are placed at 6 and 7. The light diffusely reflected from these surfaces is incident upon the cells at 8, containing solution in the upper one and

FIGURE 64. Keuffel & Esser color analyzer.

Upper, plan; lower, side view. 1, White-lined sphere; 4, wave-length scale; 5, photometer scale; 6, magnesia block; 7, magnesia block or solid sample; 8, cells for solution and solvent; 9, two-part field; 12, lamps; 14, revolving sectors; 15, motor; 17, entrance slit; 18, collimating lens; 19, dispersing prism; 20, lens; and 21, eyepiece diaphragm.

solvent in the lower. The photometric device consists of two sectored disks, one larger in diameter than the other, encased in a housing, 14, with windows through which the light passes to the slit. These disks rotate around the same axis in the same direction and are driven by a motor at sufficient speed to eliminate flicker. The smaller sector is movable concentrically across the openings of the larger while rotating, by means of the knurled head to which the photometer scale, 5, is attached. Near its edge the larger sector, intercepting the upper beam in which the solution is placed, transmits a constant amount of light which enters the upper half of the collimator slit of the spectrometer. The combined opening of the two sectors below the edge of the smaller intercepts the lower beam transmitted by the solvent, the amount of light entering the lower half of the slit being variable from 0 to 110 percent.

The spectrometer is similar to that already described in (a) and (b). Wave-length selection is accomplished by turning the knob attached to the wave-length scale, 4. The emergent spectra from the dispersion prism are directed along the telescope tube and are viewed through the ocular slit, 21, where a circular 2-part field with horizontal dividing line is seen. The two halves of the field may be matched by turning the knurled head carrying the photometer scale and the transmittancy is read directly. The lever at 13 operates an adjustment which lowers the sectors and doubles the amount of light transmitted. This arrangement is used for low transmissions, the sectors being so constructed that the scale then reads four times the actual value.

4. APPARATUS FOR ABRIDGED SPECTROPHOTOMETRY
(a) IVES TINT PHOTOMETER [14]

This instrument, which has been widely used in the sugar industry, is illustrated in figure 65. Light from the Daylite lamp located at

the back near the top is diffusely reflected upward by the block of MgCO, that rests on an adjustable support and is again reflected by the inclined mirror into the two entrance apertures of the instrument. The width of one aperture is fixed, while the width of the other may be varied measurably by a lever shown at the top of the instrument. The index on this lever traverses a scale graduated to read from 0 to 100 in equal divisions corresponding to the percentage of opening. At a reading of 100 the two halves of the eyepiece field should match. Three color filters (red, green, and blue) supplied with the instrument permit measurements to be made in the three corresponding spectral regions. A fourth filter, transmitting a rather broad band of yellow green, is provided for the use of sugar technologists. Between the mirror and the apertures, provision is made for placing absorption cells. To make a measurement, the cell containing solution is placed over the fixed aperture, while the cell with solvent is placed

over the variable aperture. With a chosen color filter in place, an intensity match is obtained by means of the lever, and the scale reading is noted.

A mercury-arc light source with appropriate spectral filters, as described under 5 (e) p. 324, may be used to advantage with the Ives Tintometer, or a Brewster or Gibson filter may be used for direct measurement at 1560 mμ (see 4, (c), p. 314).

Meade and Harris [15] evolved a method for translating the scale. readings of the Ives Tintometer into sugar color units. They obtained transmittancy readings with different concentrations of a certain raw sugar in water solution and found that any set of readings in a series is related according to a power function such that where

then

y=any scale reading,

K=the scale reading for 1 unit of material, and

x=the number of units of material required to give a scale reading of y,

y=K*, or log y=x log K, from which x=

log y

log K

To avoid repeating the calculation for each reading, Meade and Harris adopted the reading 99 as a standard and calculated a table by means of the last expression, wherein is given for each scale reading, y, the corresponding number of color units, r, of the hypothetical solution (K=99).

(b) MODIFIED STAMMER COLORIMETER

Bates [16] and associates [3] first used the mercury arc as a light source for sugar colorimetry in connection with a modified Stammer colorimeter. The arrangement of the apparatus is shown in figure 66. A sector is mounted on a shaft which rests on bearings in such position that, when rotating, the blades of the sector alternately intercept and transmit the beam of light entering the open tube of the colorimeter. When the speed of rotation is high enough to eliminate flicker, the light transmitted is directly proportional to the aperture of the sector, which may be determined mechanically. Two such sectors are used, one transmitting 80 and the other 46 percent, and are easily interchangeable on the shaft. The shaft is provided with a pulley connected to the small driving motor by a belt.

The column of solution in the closed colorimeter tube is varied in length by means of the plunger until the intensities of the two halves of the field match. The height of the liquid column, in centimeters, is read on the scale. The transmittancy, T, is then equal to the known T of the sector, from which log t is calculated by -log t=1/cb (-log T).

--

The reflection losses caused by the end plates in the solution tube and plunger are compensated by introducing two similar plates over the upper end of the open tube.

Spectral filters for isolating the yellow, green, and blue-violet mercury lines are described under 5 (e), p. 324.

(c) DUBOSCQ COLORIMETER

A standard type of Duboscq colorimeter has been adapted by Brewster [17] to purposes of abridged spectrophotometry suitable for