the saving of Iceland spar and rectilinear passage of the light through the system. In addition, great sensitivity would be secured because a practically vanishing line would be obtained between the halves of the field, as the small strip of spar need not be over 0.1 mm thick. The device was perfected and used by Bates, who subsequently succeeded in combining the two cells into one. The Brace sensitive-strip spectropolarizing system is the most sensitive yet devised. However, it is rather fragile, and its use is not recommended except in work requiring the highest obtainable precision.

(c) TESTS OF POLARISCOPES WITH CIRCULAR SCALES

Polariscopes with circular scales will be accepted for test (see test-fee schedule 421, p. 553). A thorough examination is given all optical parts. The scale will be checked for as many points as desired.

(d) NATIONAL BUREAU OF STANDARDS EQUIPMENT

Polariscopes with circular scales for measuring absolute rotations are now made by most polariscope builders. The National Bureau of Standards is fortunate in having a large Schmidt and Haensch precision instrument with a silver scale reading to 0.001°. The Lippich polarizing system is unusually good, the larger nicol having an available opening of 15 mm. The instrument is mounted on a cast-iron base 1 m in length. Owing to the necessity for accurately controlling the temperature while measuring rotations, a large air thermostat, consisting of a wooden box 40 by 55 by 60 cm covered with asbestos, is mounted between the polarizing and analyzing systems. Access is had to the interior by means of a small door in the side. The room in which the instrument is located is thermostated at a temperature approximately 1 degree lower than is desired in the polariscope air bath. An electric heater then brings the bath temperature up to the proper point and maintains it there. The heater is made of fine resistance wire wound around a large framework which fits inside the box. See figs. 26 and 27, p. 100-101. The current is controlled by means of an electronic relay operated by a mercury contact, which in turn is operated by toluene contained in a series of glass tubes so constructed and placed as to give a maximum change of volume in a minimum of time when a small temperature change takes place. The air is kept constantly stirred by a small fan. The temperature remains constant to 0.01° C. No mechanical relay of any kind is used, and consequently there is no trouble from relays sticking.

A large Weiss electromagnet, figure 8, equipped with a suitable polariscope is available for the study of magnetic rotation. This magnet is cooled by water circulation, thereby permitted continuous use even when heavy currents are employed.

2. LIGHT SOURCES FOR CIRCULAR-SCALE POLARISCOPES

(a) GENERAL

In accordance with the rapidly increasing use of polarimetry in commercial and scientific work, there has arisen a demand for greater accuracy. The largest source of error in precision measurements is in the light sources. The production and utilization of suitable light sources is by far the most difficult problem with which the polari

scopist must cope, and it therefore receives continuous study at this Bureau. For many years any sodium source was considered suitable for this work. Then came the so-called light filters of Lippich [22] and Landolt [23]. These filters, however, are open to two severe criticisms: (1) The efficiency of the purification is a function of the intensity of the source, and (2) the available light is reduced by absorption in the liquids used in the cells.

Subsequently, spectrum filtration came into use for precision work. In this method, light from an intense source is passed through an optical system containing a dispersing medium, and only the desired wave lengths are permitted to enter the polariscope.

(b) SODIUM

(1) FLAME.-Until recently the sodium lines have been the one intense source with which a large percentage of the precision work has been done. This source has been used for determining practically all of the polarimetric constants, including the standardization of quartz control plates. Unfortunately, the two sodium lines are difficult to separate from the remainder of the spectrum, and as the flame is intense there is danger of one or the other of the lines reversing. In 1906 [24] a careful study of spectrum lines as light sources for polarimetric work was made at this Bureau. It was found that the lack of intensity, the high dispersion necessary for purification, the presence of other lines of considerable intensity in the neighborhood of D, and D2, as well as the unstable line structure under certain conditions, render this source far from satisfactory. Owing to the great precision