Page:Journal of the Optical Society of America, volume 30, number 12.pdf/85

 As a piece of glass is moved through the beam, a record is made on an adding machine tape six inches in width. The advantage of this method is due chiefly to the fact that the telescope is not required to have a resolving power as high as the angular deviation which it measures. This comes about because the photoelectric cell is able, with this arrangement, to determine very accurately when the diffraction pattern produced by the objective has been bisected by the dividing line of the photometric field. The telescope objective used in these tests had a focal length of one meter, and it was fitted in most cases with a diaphragm having a circular aperture only 0.1 inch in diameter. The uncertainty in the setting was of the order of ±2 seconds. This optical system could readily be adapted to such purposes as the guiding of an astronomical telescope or the recording of galvanometer deflections.

15. Optical Properties of Evaporated and of Burnished Vitreous Quartz in the Extreme Ultraviolet. , Tufts College.

The specular reflectivities of quartz evaporated onto crystalline and onto vitreous quartz, and of burnished (slowly polished) vitreous quartz have been measured at 45°, 60°, 75°, and 85° incidence within the wave-length range 910A to 1436A. Values of refractive index and extinction coefficient have been computed from these reflectivities. These results will be compared with similar data for crystalline quartz and for vitreous quartz etched in KOH, which, having almost no surface layer, are as near “ideal” surfaces as possible. These data indicate that a burnished surface of vitreous quartz does not closely resemble an ideal surface of either crystalline or vitreous quartz, while one produced by evaporation is very unlike either ideal surface. The strong selective reflection at 1190A and the weaker one at 1070A, characteristic of both crystalline and etched vitreous quartz, are greatly reduced for vitreous quartz by burnishing the surface. The extinction coefficient curve for a typical burnished surface is fairly smooth, with the hump at 1190A only one-third as high as for etched vitreous quartz. The surface produced by evaporation shows practically no selective reflection at all whether condensed on crystalline or vitreous quartz. The extinction coefficient for evaporated quartz rises rapidly from near zero at 1436A to a value of 0.6 at 1216A. From this wave-length to 1026A it runs practically constant. As a check on this work and on the reflection method for determining the extinction coefficient, the transmission values of an evaporated surface of quartz have been measured directly to 1100A by using LiF as a support. These are in agreement with the extinction coefficient curve as determined by reflection.

16. Measurement of Numerical Aperture. , Bausch & Lomb Optical Company.

The customary definition of numerical aperture is adequate, and techniques of measurement are well known in the case of well corrected systems such as microscope objectives. In dealing with uncorrected or poorly corrected lenses such as condensers, there are deficiencies which may lead to misunderstanding and disagreement. Techniques of measurement approximating conditions of most frequent use are recommended as standards. It is suggested that condensers be tested by comparison with objectives of known N.A., using an unrestricted source for uncorrected condensers and a source which will just fill the objective field for corrected condensers. Mention is made of the term “Aplanatic Aperture” used by Carpenter-Dallinger and a suggested revision of definition is proposed.

17. An Improved Radiation Pyrometer. , The Brown Instrument Company.

In the development of a new radiation pyrometer, the following characteristics were required: (1) constancy of calibration with different distance-to-target diameter ratios up to twenty to one; (2) freedom from ambient temperature errors; (3) freedom from transient errors while ambient temperature is changing; (4) reasonably high electromotive force; (5) practically complete response within from two to four seconds. The thermoelectric type was found to be the most adaptable and dependable for general use. Consideration of ambient temperature effects, which is of much importance in modern industrial practice, was carried out by aid of mathematical analysis. In one assumed case, with a very sensitive type of thermopile losing heat from its hot junction by radiation alone, it is shown that an increase in operating ambient temperature of 180°F (from 80°F to 260°F) results in a drop in output voltage of 38 percent for a constant furnace temperature of 1300°F. The corresponding error in reading is 315°F. At a furnace temperature of 3000°F the decrease in voltage is 17 percent and the error in reading is 387°F. Under similar conditions with a less sensitive thermopile having a conduction factor of 3×10$10$, this 180°F increase in operating ambient temperature would result in a decrease in output voltage of from 12.2 percent to 12.5 percent for any furnace temperature between the two values mentioned. The error in reading would range from 58°F to 162°F over the stated span of furnace temperatures. The constancy of ratio of e.m.f.’s with the latter case makes that case suitable for compensation by means of a nickel wire shunt connected across the thermopile terminals. This method was adopted. Other methods of compensation are considered. Test data show the degree of perfection realized in the various respects indicated. Special features of construction are shown, rendering the pyrometer practically free from the usual transient errors accompanying changes in the temperature of the pyrometer. All of the thermopile junctions are spot-welded and the construction is arranged throughout to withstand high ambient temperatures.