in mutually perpendicular planes and enter the analyzer, H, a Glan Thompson prism fixed in a rotatable sleeve to which is attached the graduated circle, U. By rotation of the analyzer the intensities of the two beams may be equalized and the angle of rotation noted on the circular scale. The light of the two beams transmitted by the analyzer is focused upon the biprism at I, whereby it is directed along the axis of the photometer. On leaving the analyzer, the light of these two beams is linearly polarized in the same plane, so that the action of the biprism on each beam is the same.

At the eyepiece diaphragm, K, may be seen a circular field with horizontal dividing line formed by the edge of the biprism which is brought into focus by means of lens, J. By rotation of the analyzer through 360°, it is seen that complete extinction of each half of the field occurs twice at points 90° apart. Midway between each pair of extinctions lies a point at which the two halves match. This is the zero point. When the instrument is in proper adjustment and both apertures are equally illuminated, the zero point falls at 45° and the photometer becomes direct reading. 15 One quadrant of the circle is calibrated to read directly in terms of T, a second quadrant in terms of log percent, and the remainder of the circle in degrees from 0° to 180° C.

The eyepiece diaphragm at K is replaced by a short-focus lens that images the photometric field upon the adjustable vertical collimator slit of the spectrometer at S in the focal plane of the collimator lens, 01. Spectrally dispersed by the prism, P2, the light enters the telescope system through the objective, O2, and is observed through the eyepiece lens at d. Within the eyepiece holder, R, is a shutter or eyepiece slit, the width of which is adjustable by lever b, thus defining the length of the spectral band observed and controlling the spectral purity. The eyepiece slit is movable horizontally across the axis of the telescope and along the length of the spectrum by turning screw head a, which provides a means of wavelength calibration. The dispersing prism is mounted on a turntable actuated by a micrometer screw to which the wavelength drum, WD, is fixed. The latter is graduated in units of 1 mu, from 400 to 800 mμ. The rotation of the prism table brings any desired part of the spectrum upon the central part of the field as defined by the ocular slit and as indicated on the wavelength drum. At the eyepiece lens of the spectrometer the observer sees a rectangular field composed of two juxtaposed spectra with horizontal dividing line; in other words, a portion of the spectral image of the two-part photometric field.

The wave-length scale of the spectrometer may be calibrated by means of mercury or helium lamps, or by wave lengths from other sources. The photometer scale may be checked by means of rotating sectors of fixed opening (see following section) [10], which may be accurately calibrated mechanically, or by means of filters of accurately known transmission [9]. Measurements on a solution at different thicknesses may also assist in detecting errors, the negative logarithm of the transmittancy being accurately proportional to the thickness [9].

15 In earlier designs of the Martens photometer, the biprism was cemented to the front face of the Wollaston prism. The light of the transmitted beams, being polarized in planes mutually perpendicular, was affected unequally by the inclined faces of the biprism, and the amount of light transmitted by the two halves of the biprism was not the same. The zero point, therefore, did not fall at exactly 45°, which precluded the use of direct reading.

A pair of absorption cells to contain solution and solvent, respectively, are shown at C. The diameter of these should be such that light does not reach the inside cylindrical surfaces and become reflected into the photometer. This, however, may be prevented by using proper diaphragms. The focusing lenses at B, nearest the photometer, may be omitted in many cases or may be replaced with a diaphragm, thus allowing longer absorption cells to be used. To permit measurements over a wide range of transmission and long spectral range, an assortment of matched pairs of cells should be available in thicknesses between end plates of 0.5, 1.0, 2.0, 5.0, 10.0, and 20.0 cm. The 0.5, 1.0, and 2.0-cm sizes are available in the usual Ushaped form with glass separators, while the larger sizes are in the form of glass cylinders fitted with screw caps in the manner of saccharimeter tubes and having stoppered side openings. Cells with fused-on glass end plates may now be purchased.

The interchange method is used in measuring the transmission of a solution relative to that of the solvent at any wave length. After a series of measurements has been recorded, the position of the cells relative to each other (and to the respective apertures of the photometer) is reversed and a second series of an equal number of readings is taken. If either of the direct-reading scales has been used, the arithmetic mean of all the readings is taken as T or-log Tx. If the degree scale is used with the zero point falling at or near 45°, the arithmetic mean of each series of readings in angular degrees is taken and the transmittancy is found by

T=cot 0,Xtan 02,

(89) where 0 and 2 are the observed angles of match, respectively, with the sample in positions 1 and 2.

By the use of an illuminated white-lined sphere substituted for L, the apparatus may be used for spectral-reflection measurements. The reflection standard and the sample are in a rotary specimen holder in a vertical position at the side of the sphere opposite the apertures of the photometer and are illuminated by the white inner surface of the sphere which contains two 500-watt Mazda lamps. Soft sugars may be pressed into a suitable container in a manner to present a smooth, flat surface, the reflectance of which may be measured. For dry granular sugars a cover must be used to retain the sample because of the vertical mounting. According to Judd and Gibson [56], an error of as much as 10 percent may result from the use of the cover glass.

(b) GAERTNER POLARIZING SPECTROPHOTOMETER

This apparatus, figure 62, also consists of a spectrometer, polarizing photometer, illuminating device, and equipment for mounting the samples [12].

The white-lined diffusing sphere at the right contains four 200-watt incandescent lamps and serves for illumination. For transmission measurements, a diffusing white surface is placed at the back of the sphere and serves as the light source (upper drawing). This may be replaced by an opal or ground glass, if desired, permitting illumination by a mercury arc or other source. Light from the diffusing surface is collimated by a lens and then divided by a pair of rhombs into two

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

OPTICAL PATHS FOR REFLECTION MEASUREMENTS

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

FIGURE 63.-Königs-Martens spectrophotometer.

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, Ro, is given by the relation, analogous to formula 89,

RS
=cot 0, tan 02.
Ro

(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.