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line in series with a suitable rheostat only. Its shape makes it particularly adaptable for use in front of a collimator slit. The General Electric lamp is the more suitable where a broader source is required. Neither lamp gives light consisting of the two sodium D lines alone. While in the light there is very little of the continuous spectrum background which is always present in flame spectra, yet these sources cannot be used without proper purification, as there are four other doublets in the sodium arc spectrum which are ordinarily not observed in the flame spectrum, as shown in table 3:

TABLE 3.-Wave lengths (in angstroms) of lines in the arc spectrum of sodium [The figures in parentheses refer to the relative intensities of the lines as given in "Lines in the Arc Spectrum of the Elements" [31]]

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In many cases light filters may be used successfully, but where precision rotation measurements are required, spectral purification is needed.

The rigorous requirement as to purity may to some extent be realized when it is known that a quartz plate, which reads approximately 40.000° for D1 (5896), will read about 40.100° for D2 (5890). If one is attempting to read to 0.001°, or even to 0.01°, it is obvious that the purity of the light is of much greater importance than has generally been realized.

(c) MERCURY LINES

For the production of the so-called yellow-green line of mercury (λ=546.1 mμ) several types of lamps are available. In all of them, advantage is taken of the fact that mercury vapor, heated to incandescence by an electric current in a vacuum, gives an intense line spectrum. The most important lines, in Angstrom units, and their relative intensities are given [31] as 6908 (10), 5790 (10), 5769 (10), 5461 (10), 4916 (6), 4359 (10), 4078 (8), 4047 (8). Some type of optical dispersion system must be resorted to in order to separate the desired line. At this Bureau the fused-silica mercury lamps originated by Heraeus are used. They may be obtained in almost any desired shape. The straight Uviarc types are particularly convenient. Owing to the relatively high melting point of fused quartz, they can be safely operated at high intensities without water-cooling. Great care must be exercised not to permit the radiation from these lamps to enter the eyes without first passing through protective glasses.

The yellow-green line of incandescent mercury vapor, λ=5461, was proposed by Bates [24, p. 243] as the standard source for all accurate polariscopic work. Quartz mercury-vapor lamps as now made are reliable in action. If sufficient care is exercised in preparing the mercury and exhausting the lamps, the characteristic mercury lines only will be obtained in the visible spectrum and with great intensity. Different observers have found markedly different line structures for the line λ=5461 A, depending upon the method of analysis employed. The map in figure 11 was obtained with the echelon with 1.8 amperes passing through the lamp. The fractional values given are

the relative intensities as nearly as they could be estimated. It was found impossible to decide whether the satellite, -0.24, belongs to the positive or negative side of the primary. When the current was increased to more than 2.1 amperes, the satellite, -0.55, increased in intensity until it about equaled the primary. The difference in wave lengths of the extreme satellites is less than 0.4 A. With a different source, Fabry and Perot [32] found 0.35 A and Houstoun [33] 0.215 A. The distance between D, and D2 of sodium is 15 times 0.4 A. as it has been possible to determine, the line structure for the quartz lamp, under widely varying conditions, is such that for polariscopic purposes, λ=5461 A is a monochromatic source of great intensity and perfect reliability. In measuring a rotation of 250°, no differences could be detected due to changes in the emission, and probably none for much larger rotations. The quartz lamp requires little attention and can be operated indefinitely. Since only the lines λ=5790, 5769, 5461, 4358, 4078, and 4047 A are important, the difference in wave length is such as to permit of perfect separation of the line 5461 A by even a relatively small dispersion, and without bringing other lines in

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FIGURE 11.-Map of the yellow-green mercury line, λ=5461.

close proximity to the edges of the slit. Hence this source permits the elimination of practically all diffused light.

Since the adoption of X=5461 A as the official source for polarimetric measurements by the National Bureau of Standards, its universal acceptance has been recommended by a number of investigators. This Bureau urges its general use, especially when measuring circular degrees in research and precision work, to the end that reliable comparisons may be made between the results of different investigators.

(d) CADMIUM LINES

(1) GENERAL.-The necessity for a number of suitable line sources for polariscopic work has resulted in numerous investigators giving

FIGURE 12.-Mercury vapor lamps.

A, Shows the original quartz Heraeus lamp first shown in this country at the St. Louis Exposition in 1902. It is still in good operating condition.

B, Shows a group of modern mercury-vapor lamps. In the foreground is a horizontal 110-volt direct-current Uviarc burner; on the extreme right is the same type designed for operation in the vertical position; on the extreme left is a 400-watt alternating-current lamp in glass, which is almost as satisfactory for polariscopic work as the more expensive quartz burners. In the middle are the 1,000-watt, water-cooled, highpressure quartz capillary lamp (left) and the new 600-watt alternating-current type Uviare employing only a slight amount of mercury, all of which is vaporized in operation [34].

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much time to the subject. So far as possible, it is desirable that the lines utilized be uniformly distributed throughout the visible spectrum. Lowry [35] has suggested the following (given in angstroms): Lithium, 6708, red; cadmium, 6438, red; sodium, 5893, yellow; mercury, 5461, green; cadmium, 5086, green; cadmium, 4800, blue; mercury, 4539, violet. 6708 and 5893 were obtained from flame spectra, and 5461 and 4359 from the quartz-mercury lamp. The cadmium lines 6438, 5086, and 4800 Lowry suggests be obtained from a rotating arc. The electrodes must rotate in opposite directions at a speed sufficiently high to prevent flickering. As electrodes, he uses an alloy of 28 percent of cadmium and 72 percent of silver. The melting point is 860° C. This method has been used at this Bureau and is capable of giving excellent results. The rotating arc is, however, rather difficult to manipulate. Some of the silver lines, namely 5469,

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5209, and 4208, may be obtained in this manner also, and with sufficient intensity for polarimetric measurements.

(2) BATES CADMIUM-GALLIUM LAMP [36]. In order to improve the cadmium source, this Bureau has developed a vacuum type of cadmium arc. If pure cadmium is used in a quartz lamp, the adhesion between the cadmium and the quartz results in the destruction of the lamp upon the solidification and cooling of the cadmium. If the cadmium is mixed with mercury to make a soft alloy, the cracking of the lamp is effectively prevented, but the mercury vapor then carries most of the current, since the vapor pressure of mercury is much greater than that of cadmium.

On the other hand, gallium although melting at about 30° C, boils at a very high temperature, approximately 1,500° C. Its vapor pressure is therefore negligible in comparison to that of cadmium. Furthermore, the addition of a few drops of gallium to 10 or 15 ml of cadmium is found to change completely the texture of the latter, rendering it relatively soft and greatly reducing its tensile strength. Subsequently, it was discovered that upon distilling the cadmium from the alloy at a pressure of about 0.001 mm of mercury, the minute quantity of gallium carried over was sufficient to change completely the character of the cadmium and to prevent adhesion between the cadmium and the walls of the lamp. The type of lamp usually used is shown in figure 14.

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The total volume of the lamp was approximately 10 ml. The electrodes consisted of tungsten wires, B, entering through quartz capillaries. These were closed with seals similar to the type described

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FIGURE 14.—A, Bates type of cadmium-gallium arc; B, gallium spectrum.

1, Cadmium-gallium alloy; B, tungsten electrode; C, lead seal: D, tungsten lead wire: E. capillary for scaling off lamp from pump.

by Sand [37], and later by quartz-Pyrex-tungsten seals when these became available.

In filling the lamp, cadmium containing 2 or 3 percent of gallium, is placed in the side tube, F, and distilled under a vacuum of 0.001 mm Hg, or better. When carefully prepared, the lamp will have an indefinite life. One of this type has been in intermittent use for several years and shows no sign of deterioration. The lamp may be

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