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 MgCO3 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 X560 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

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

y=K1, 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) DUBOSCO COLORIMETER

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

sugar factory laboratories. The light source may be either a mercury are with the appropriate filters or an incandescent tungsten lamp with which a special 560-mu filter is used. Glass plates calibrated for transmission at the various wave lengths are used as standards. The apparatus, figure 67, consists of a Bausch & Lomb Duboscq colori

FIGURE 66.-Stammer colorimeter modified for abridged spectrophotometry.

meter with 10-cm cups, to the vertical column of which a shelf with a heavy bracket is fastened with screws to leave a clearance of 1 cm when the cups are in their lowest position. A carrier for the standard plates with a 25-mm center orifice, and with the ends cut as shown, slides between guides on the shelf and has a slot that engages in a

metal stop at the center of the shelf to center properly the orifice under either cup. A 60° prism enclosed in the triangular housing of the eyepiece telescope permits observation with the head in a natural, unstrained position. The adjustable mirror at the bottom of the colorimeter is silvered glass with the outer surface ground to diffuse the reflected light.

Spectral filters for obtaining light from ordinary tungsten illumination at wave length 560 mp or close thereto have been devised by Brewster [18] and by Gibson [19]. These permit very satisfactory measurement of transmittancy at this wave length. The composition of these filters is given in table 38.

The components are sealed together with Canada balsam and may be bound with black tape or mounted in metal. The filters may be cut to such diameter as to be mounted in the telescope tube of the instrument or may be used over the eyepiece. In the Brewster filter the gelatine sheets are sealed between the glass components. In the Gibson filter the Jena glasses are sealed between the Corning components.

In the Brewster filter the spectral centroid of the transmitted light for incandescent tungsten illumination at 2,848° K was computed to be 558.8 mu. For the original Gibson filter and certain replicas made at this Bureau, the optical centroid, under the same conditions as above, was found to be located at 560.0 mμ.

16

A second filter consisting of a blue glass, Jena BG-12, thickness 3.42 mu, is used by Brewster for the special case of very pale sugar solutions, such as those of high-grade refinery products. The optical centroid of this filter was computed for 2,848 K as being at 459.9 mμ.17

The photometric standards are plane parallel plates of polished glass, the color of which is known as "carbon amber." This glass is obtainable in shades varying from brown to palest straw. These are to be calibrated in terms of transmission, T, at wave length 560 mμ. This calibration is best obtained by sending the plates to the National Bureau of Standards with a request for calibration, as above specified. The transmission value may be controlled by the shade and thickness of the glass, and it is advisable to have plates of three values,

16 At the time this was written there was difficulty in obtaining exact duplication in the two Jena glasses used in the Gibson filter. However, it is doubtful that a slight shift of the optical centroid on either side of A560 by 1 or 2 m would greatly affect the results in sugar colorimetry. Attempts to duplicate this filter by substituting glasses with transmission properties too greatly different from those specified by Gibson would likely result in an inferior filter.

17 In order to measure transmittancy in extremely pale sugar solutions at 560 mp, excessive layer thickness is required beyond the 10 cm available in the standard type of Duboseq colorimeter. Employment of the 460-mu filter is therefore a compromise between having an instrument constructed with much longer cells or choosing a wave length at which the light absorption is greater.

one each with Tx-560 .=560 mu about 50, 70, and 80, respectively. This makes for flexibility of the method, permitting measurements over a sufficient range of shades. The glass is customarily supplied in the form of 2-inch polished squares, but other shapes and sizes may be specified to conform to the construction of the instrument. For measurements with pale solutions and the 460-mu filter, a plate consisting of colorless optical glass calibrated in terms of T at 460 mμ is used. To measure the transmittancy of a solution a portion is added to each colorimeter cup, a standard is placed in the carrier and centered under one of the cups, which is adjusted in height to a scale reading of 1 cm. With the spectral filter

in place, the observer adjusts the height of the opposite cup to a photometric match and records the scale reading. Three to five such settings are made, and the standard is shifted by sliding the carrier and centering the standard under the opposite cup, which is then adjusted to the 1-cm scale reading as above, and a second series of the same number of readings is recorded. This substitution method of comparison is the usual one employed with direct-reading spectrophotometers and serves to compensate for any error of zero setting.

The readings in centimeters are added, and the sum is divided by the total number of readings to give the mean. The blank setting (1 cm) is deducted, and the remainder represents that thickness, b, of the solution whose transmittancy, T, equals the calibrated transmission of the standard at the specified wave length. Assuming that the dry substance concentration, c, in grams per milliliter, as given in table 114, (corresponding to the refractometric Brix of the

FIGURE 67.-Duboscq colorimeter with accessories for abridged spectrophotometry.

solution) is already known, the final value, log t, is found by substituting the values of b, c, and log T (table 129) in the LambertBeer equation, log t=1/cb (-log T).

(d) ZEISS-PULFRICH PHOTOMETER

This instrument, with accessories and methods of manipulation for various photometric purposes, was first described by Pulfrich [20]. The diagram in figure 68 illustrates the photometer (accessories not shown). The optics will be understood from a glance at the drawing.