solution, leaving only that part of the spectrum in which S-Q is negligible in the case of sucrose. In the case of dextrose this is true

to a lesser extent.

Owing to the fact that the bichromate absorption cell is indispensable when a white-light source is used and that a suitable and convenient form has not been available, the type shown in figure 20 has been designed at this Bureau especially for saccharimetric use. The cell is entirely of glass with an inner separation of the plate-glass walls of 15 mm. It is mounted in a metal frame carried on the stand, F. The height is adjustable with ample range to fit any saccharimeter. To utilize the filter it is only necessary to fill the cell with a 6-percent.

50

40

440 460 480 500 520 540 560 580 600 620 640 660 680 700

WAVE LENGTH

FIGURE 19.-Spectral transmission curves for bichromate filter.

Comparison of spectral-rotation curves for the normal quartz plate and normal sucrose solution, and the cutoff of these latter curves by the bichromate filter.

solution of potassium bichromate and place it between the light source and the polarizer, preferably as close to the latter as possible. Recently a new mount has been designed for this cell and built as an integral part of the lamp housing (fig. 21), thus doing away with the need for the separate stand shown in figure 20.

Colored glasses whose absorption very closely approximates that of the 1.5-cm layer of a 6-percent bichromate solution are now available commercially. These are far more convenient to use and serve admirably to accomplish the same primary function as the bichromate solution, namely the removal of the blue end of the spectrum. However, they are open to the possible objection that they are not so efficient in removing heat rays from the lamp as is the water solution. When they are used, it may be desirable in some instances for precision

Bausch & Lomb Optical Co., Rochester, N. Y.; Corning Glass Works, Corning, N. Y.

S-Q

work to use a plain water cell or a heat-absorbing glass between the lamp and saccharimeter to prevent heat rays from being absorbed by the solution under test. Should colored glasses be used, care should be taken to make sure they actually have the same spectral transmission as the specified bichromate solution. Instances have been observed where saccharimeters sent to this Bureau for test were equipped with

FIGURE 20.-Bichromate filter and stand for saccharimeters (NBS design). 4, body of glass cell; B, B', metal cell holder; C, washer; D, glass end plates of cell; E, ground-glass stopper; F. stand and locking device; G, threaded cap; S, solid rod.

colored glasses, of foreign make, which did not even approximate the bichromate solution in their spectral characteristics.

(c) INFLUENCE ON READING

In 1904 Schönrock [16] made an investigation of the effect of different light sources on the saccharimeter reading. His results are summarized in table 6.

The data in table 6 were obtained with uncolored sugar solutions. It will be observed that with a white-light source the presence of the bichromate absorbing solution makes a difference of 0.12° in the saccharimeter reading. It thus becomes important that the instrument be used under the same conditions as prevailed at the time it was standardized. If, as is usually the case, a quartz control plate is used, the plate should be read in the instrument under the same conditions as prevailed when the plate was standardized.

(d) TYPES OF LAMPS

(1) MONOCHROMATIC.-Many different types of lamps designed for use with the saccharimeter are available. They may be divided into two classes those giving a monochromatic or nearly monochromatic light and those giving white light. In the first class only the yellow sodium lines and the yellow-green mercury line (λ=5461 A) have been utilized to any extent and these usually for special purposes. Sources of this type must be used for circular-scale polarimeters; they are in general not satisfactory with saccharimeters which were designed for the more intense white-light source.

(2) WHITE LIGHT.

Gas. A considerable variety of lamps suitable for white-light sources is available both for gas and for electricity. Most of the gas lamps utilize the Welsbach mantle, which is the source formerly most generally used in saccharimetry. The light is convenient, has considerable intensity, and the radiating surface has a nearly uniform intensity over a sufficiently large area. A ground-glass screen may be used close to the mantle if desired, and is necessary if the lamp cannot be so placed as to eliminate a mottled appearance of the field when the telescope is in focus for the analyzer diaphragm.

Electric. The available electric lamps are of several types. A ground-glass disk, which becomes the new source of radiation, must, with few exceptions, be used with all types, and is preferably located as near the radiating surface as the temperature will permit. In figure 21 is shown an electric lamp developed at this Bureau, which has recently been modified to carry the bichromate filter. The concentrated-filament incandescent stereopticon lamp for 110 volts is used. The area illuminated is ample for the purpose, and the intensity sufficient. Convenience has been the chief consideration in the design. The base B is heavy. The ground-glass disk, R, 38 mm in diameter, is easily removable and is adjustable vertically with respect to the body, J. Thus the filament and the disk may be kept

The maker's identifying specifications for this bulb are G. E. Mazda, clear spotlight, 100 watts, 110 volts, C5 filament, P25 bulb, medium screw base.

FIGURE 21. Saccharimeter lamp with bichromate cell holder (NBS design).

A, base perforated for ventilation; B, cast-iron stand; C. hollow post; D, pinion for adjusting height of lamp: E, set screw; F, solid post fitted with rack; G, glass cell; H, porcelain lamp socket; I, removable disk light screen; J, lamp housing fitting into flange, K; L. adjustable slide; M. removable cover held by springs, N: O, side tube carrying ground glass and bichromate cell; P, binding posts; Q, insulating blocks; R, flange to hold ground-glass plate; S, slot opening for ventilation.

centered, giving uniform illumination of the disk. The cap, M. permits the heat to pass off but no light to escape into the room, The height of the lamp is regulated by the rack and pinion, D. The center of the disk can thus be accurately set in the axis of the optical system of the saccharimeter. The electric connection is made at the binding posts, P. This lamp has proved very satisfactory in the laboratories both of this Bureau and of the United States Customs Service and is the type most highly recommended for general use. The firm of Schmidt & Haensch has taken advantage of the small size of the 6-volt lamp to mount it in an attachment which fits the metal housing containing the polarizing system of their saccharimeter.

Left, tungsten arc in vacuum (30-ampere arc); right, ribbon-filament lamp (6 volts, 108 watts).

The heat developed so near the optical parts is objectionable. The illumination is only fairly satisfactory when the lamp is new, and the efficiency in most cases decreases rapidly with use.

In special cases where the 100-watt lamp does not yield sufficient light, other sources have been utilized.

The Nernst glower was useful for some years but is now of little more than historical interest. It has been largely displaced by incandescent stereopticon bulbs of higher wattage than that described above. They are obtainable in 250-, 500-, and 1,000-watt ratings.

Other high-intensity sources are (1) (fig. 22 right) the 6-volt, 108watt ribbon filament lamp, which is operated from a small transformer; and (2) (fig. 22 left) the tungsten are in vacuo developed by