Babinet Compensator Research Articles

New methods in the study of light scattering

A method is described, based upon the use of a Nicol for polarising the incident beam in any desired azimuth and of a Babinet compensator for observing the transversely scattered light which enables the Krishnan effect to be very conveniently studied. The sensitiveness of the arrangement permits its use for the observation of the effect in cases where the methods previously employed are not delicate enough. It is pointed out that the magnitude of the effect would be enhanced by using ultra-violet radiation, the compensator fringes being recorded photographically. The same arrangement can also be employed for detecting and measuring the elliptic polarisation of the scattered light when the particles scattering the light are uniform and spherically symmetrical. The wide field of utility of the method in the study of light scattering is indicated.

Studies of the Mechanical Properties of Substances of High Molecular Weight I. A Photoelastic Method for Study of Transverse Vibrations in Gels

The wave-lengths of transverse vibrations in gels are measured at various frequencies by observing the strain double refraction stroboscopically. Use of a Babinet compensator inclined at a small angle to the plane of polarization of the incident light detects very small amounts of double refraction. From the velocity of propagation of the vibrations, the modulus of rigidity is calculated; values ranging from 300 to 13,000 dynes/cm2 have been determined.

An improved half-shade system for the detection of elliptically polarized light

This paper describes the application of the reflection theory of Drude to the examination of surfaces. The essential characteristic of the method is that the state of polarization of the incident light is so adjusted that a plane-polarized beam is reflected. The absence of any ellipticity in this reflected beam is confirmed by means of an improved ellipticity half-shade system, with which two parts of a field of view can only be matched when the incident light is accurately plane-polarized. Two uses of this system are described. Details are given of the construction and use of the improved balanced ellipticity half-shade made from mica sheets, on the general principle of the Bravais biplate. The half-shade is shown to have a higher accuracy than a Babinet compensator.

A New Method for Measuring the Tensile Strength of Glass

Tensile strength of glass.---Lack of uniformity and reliability in the results hitherto obtained is due to the fact that the strength of the surface is greatly reduced by slight scratches. The new method of measurement here described attempts to eliminate as far as possible the effect of surface defects. A bar of glass is used which has been cut from the middle of a slab which had been heat treated in such a way as to leave the upper and lower surfaces of the bar under high compression, while the inside is under tension. When this bar is broken by bending, the tension introduced in the lower surface is largely neutralized by the initial compression, so that the maximum tension is not in the lower surface but in a zone nearer the neutral axis. The initial stresses are measured by means of polarized sodium light and a calibrated Babinet compensator, and these combined graphically with the stress due to the central load required to break the bar, give the maximum tension introduced which is taken as the lower limit to the strength of the glass. In this way values of 12 and 15 kg/${\mathrm{mm}}^{2}$ were obtained for a lead glass and a borosilicate glass, values twice as great as those given by the usual methods; and the mean deviation of individual measurements is less than half as great.

On the di-phasic nature of glass, as shown by photo-elastic observations

In the course of an investigation on the effect of temperature upon the photo-elastic constants of glass, an specimen of flint glass was heated slowly to 400º C., and then subjected to a longitudinal pressure of 130·6 bars in a straining apparatus working on the lever principle, and very similar to that described by one of us. Normally, the load had been removed immediately after readings had been taken, but on one particular occasion the full load was left on for 42 hours, and then the specimen was allowed to cool slowly under this pressure to atmospheric temperature, the cooling process taking 7 hours. The load was then remove, and the specimen examined in sodium light between crossed Nicols with the aid of a Babinet compensator according to the method described by Filon and Jessop. It was then found that the black band in the Babinet compensator showed considerable displacement from its zero position. This was only to be expected, since naturally internal stress should be set up and the glass disannealed under these conditions.