Savart Plate Research Articles

Sets of Cascaded Plane-parallel Birefringent Crystal Plates

A solution is presented which makes it possible to consider separately the effect of the path differences and the directions of polarization on plane wave fronts. It is shown that the complicated pattern observed behind a polarizer placed after a set of n parallel birefringent plates can be separated into ½(3 n − 1) systems of fringes (line patterns). Each of these patterns is represented by a separate term in the equation for the intensity of the light emerging from the polarizer. The shape and spacing of the fringes is determined by the phase differences, their intensity is given by the directions of polarization. The result of the calculation is given in the form of a simple algorithm. It is applied to the Savart plate considered as a cascade of two uniaxial plates. The results are confirmed by photographs of the conoscopic pattern of Savart plates. By using an immersion objective aperture 1·3 a conoscope with an extremely wide angle of vision was obtained. Two types of Savart plates exist which are re...

Measurement of Transfer Functions of Photographic Objectives by Means of a Polarizing Shearing Interferometer

A polarizing interferometer has been constructed for the measurement of transfer functions of photographic objectives. The polarizing system of the interferometer consists of a modified Savart plate of effectively varying thickness to obtain a continuous lateral shift of the aperture of the objective and of a Soleil–Babinet compensator for the accurate measurement of the phase of the function. Fringes have also been observed whose deformations represent lateral aberrations. Comparisons were made between the experimental results and those which were obtained by geometric-optical calculations from the lateral aberrations.

A Novel Prism for Total Reflection

For purposes of autocollimation alignment a reflection prism has been developed with certain advantages over the triple mirror (corner-cube). Not only is the parallelism of the incident and emergent rays (travelling in opposite directions) independent of the orientation of the prism, but it is also possible to conserve much better the state of polarization. This makes the prism useful for the determination of the optical thickness of thin layers in vacuo by means of Savart's plates in autocollimation. Some advantages and disadvantages are discussed.

Spectral Line Broadening Measured by Shearing Interferometric Methods

A shearing interference method for determining spectral line widths and shapes from the visibility curve of two-beam interferences as a function of the path difference between the interfering wavefronts is described. The visibility is the Fourier transform of the intensity curve of the spectral line. In each measurement a limited number of points of the visibility curve is determined, but this is sufficient to give a rough estimation of the whole curve if the points are properly chosen. The image is preferably registered by a photographic plate which is then measured in a microdensitometer. Hence light sources of very low intensities can be investigated. Two slightly different shearing interferometers have been used, one with a Savart plate combined with an entrance slit and the other with a Wollaston-type compensating prism for large apertures. In the latter case, no entrance slit is necessary and combinations with a wide band monochromator can be made without any serious loss of intensity. The green line from mercury-spectral lamps has been examined, and measurements on a high pressure lamp are reported.

An Eyepiece for Measuring the Percentage Plane-Polarization in a Beam of Light

With this eyepiece the plane-polarization in a beam of light is compensated by means of a thin, plane-parallel plate of celluloid tilted at the proper angle. To ascertain if the compensation is complete a detector consisting of a quartz plate and a second tiltable plate of celluloid is used in combination with a Savart plate and a nicol or Wollaston prism. The biquartz plate consists of two plates of quartz cut normal to the optic axis, the one of dextrogyre and the second of laevogyre quartz, mounted side by side with polished junction faces. The thickness of the quartz plates is such (1.76 mm) that for mercury green light of wave-length 5461A the rotation is ±45°. An incident plane-polarized beam is divided into two beams by the biquartz plate; the plane of vibration of the beam emerging from the first half of the plate is normal to that of the second beam. The tiltable plate of the detector is mounted above the biquartz plate and has its axis of rotation in the plane of vibration of the wave emerging from one of the biquartz plate halves. By means of this second plate a small amount of polarization can be added to, or subtracted from the polarization in the transmitted beams. The Savart plate and analyzing prism serve to detect the presence of plane-polarized light in the beam. With the aid of this eyepiece the amount of plane-polarization can be determined, for low percentages, to one-fifth of one percent. The eyepiece has been used chiefly in the measurement of the percentage plane-polarization in light reflected by different parts of the moon’s surface and by terrestrial materials; also for the measurement of sky polarization and in metallographic work.

Polarisation of Light from the Sky during the Solar Eclipse of January 24

DURING the total eclipse on January 24, I was at New Haven, Connecticut, and thus very close to the middle line of the track of totality. For a portion of the period of totality I made a somewhat hasty survey of the state of polarisation of the light scattered by the sky, using for the purpose a Savart plate and Nicol prism. I was only able to cover the eastern sky from the zenith to the horizon stretching from north through east to south, and I was unfortunately not able to determine the plane of polarisation of the light scattered from the various parts of the sky. What I was able to note, however, with certainty was that there was no marked variation in the percentage of polarisation as one explored the eastward half of the sky—that is, there was no trace of the familiar maximum noted when the sun is not in eclipse.

NOTE ON SOME METEOROLOGICAL USES OF THE POLARISCOPE

This is merely a preliminary notice of certain facts regarding atmos pheric polarization which may prove to have some prognostic value. They were incidental to a proposed study of the character of autumnal haze which the writer undertook last year at Mount Moosilauke, N. H. This peak, 4811 feet high, has an almost uninterrupted sweep of horizon over a radius of one hundred miles or so and offers an excellent chance for investigating the distribution and nature of the haze that veils the landscape in early autumn. For instruments I took along a Savart polariscope, merely a Savart plate with a bit of tourmaline as analyzer, an extemporized double-image polarimeter of the type outlined in the early and valuable paper of Professor E. C. Pickering,1 a couple of carefully calibrated photographic wedges for determining opacities, and a direct vision spectroscope. A prolonged easterly storm, about the only thing which could have defeated the program, cut short observations upon the summit, but a week of preliminary observations at Breezy Point (elevation 1650 feet) at the base of the mountain yielded results which seem to be of suffi cient interest to put upon record. These were made mostly with the Savart polariscope, an instrument which, from its very wide field of view and great sensitiveness, showing even one or two per cent of polarization, enables sky conditions to be very readily investigated. The character of the sky polarization, with its general symmetry and maximum in a plane at 90? solar distance, is well known, but the nature and causes of its casual variations have not, perhaps, received the attention that is their due. Nearly everything in the landscape polarizes by reflection to a greater or less extent, the more as the specular component of reflection is the greater. For example, the glossy upper surface of a maple leaf polarizes strongly at fairly large angles of incidence, while the mat lower surface has only

Note on Some Meteorological Uses of the Polariscope

This is merely a preliminary notice of certain facts regarding atmos pheric polarization which may prove to have some prognostic value. They were incidental to a proposed study of the character of autumnal haze which the writer undertook last year at Mount Moosilauke, N. H. This peak, 4811 feet high, has an almost uninterrupted sweep of horizon over a radius of one hundred miles or so and offers an excellent chance for investigating the distribution and nature of the haze that veils the landscape in early autumn. For instruments I took along a Savart polariscope, merely a Savart plate with a bit of tourmaline as analyzer, an extemporized double-image polarimeter of the type outlined in the early and valuable paper of Professor E. C. Pickering,1 a couple of carefully calibrated photographic wedges for determining opacities, and a direct vision spectroscope. A prolonged easterly storm, about the only thing which could have defeated the program, cut short observations upon the summit, but a week of preliminary observations at Breezy Point (elevation 1650 feet) at the base of the mountain yielded results which seem to be of suffi cient interest to put upon record. These were made mostly with the Savart polariscope, an instrument which, from its very wide field of view and great sensitiveness, showing even one or two per cent of polarization, enables sky conditions to be very readily investigated. The character of the sky polarization, with its general symmetry and maximum in a plane at 90? solar distance, is well known, but the nature and causes of its casual variations have not, perhaps, received the attention that is their due. Nearly everything in the landscape polarizes by reflection to a greater or less extent, the more as the specular component of reflection is the greater. For example, the glossy upper surface of a maple leaf polarizes strongly at fairly large angles of incidence, while the mat lower surface has only

An investigation of the influence of electric fields on spectral lines: preliminary note

In general the electrical fields used were those concomitant with the luminous electric discharge. An interferometer of the Michelson form and an echelon spectroscope of 18 plates were used to analyse the radiations. The results may be summarised as follows:— (1) End-on discharge tubes of special design, in which the light-source was a uniform column of luminous mercury vapour viewed in the direction of discharge, showed no change of wave-length as great as 1 part in 4,000,000 when the direction of the discharge was reversed. The pressure in the tube was varied from a few millimetres to a vacuum so high that there was but little luminosity. (2) The passage of Röntgen rays through the tube did not alter the wave-length nor the width of the mercury lines, to an extent sufficient to affect the visibility of interference fringes formed with a difference of path of 400,000 waves. When the luminous column was viewed at right angles to the direction of the discharge no polarisation effects in the radiation from it, due to the passage of the Röntgen rays, could be detected by a sensitive Savart plate and Nicol prism.