phragm D below the polarising plate F, and in that case the eye will receive the polarised ray reflected

H

FIG. 13.

from the mirror; and the polarised ray will have passed, before it reaches the eye, twice through the crystalline plate placed between the mirror and the polariser. The result is the same as if, in the ordinary apparatus, the polarised ray had passed through a plate of double the actual thickness. If the plate does not fill the entire field of view, two images of the plate will be seen, the one larger, as viewed directly,

the other smaller, as viewed after reflexion from the horizontal mirror; the first will show the effects due to the actual thickness of the crystal, the other that due to a plate of the same crystal, but of double the thickness.

It was mentioned in Chapter I. that light might from various circumstances become partially polarised; and as this is not unfrequently the case in natural phenomena, such as the polarisation of the sky, or of the corona visible in a total solar eclipse, it becomes desirable to have some measure of the proportion of polarised light in a partially polarised beam. The following is a description of one such instrument, or polarimeter as it is called, devised by Prof. W. G. Adams.

This instrument consists of a box, about 4 inches square and 2 inches deep, inside which four parallel plates, very little smaller than the cross section of

FIG. 13A.

the box, are attached to an axis passing through the middle points of the square faces of the box. On one of these faces is a circle graduated to degrees, and

an index attached to the axis and parallel to the planes of the plates passes over this circle when the axis is rotated, and shows the inclination of the plates to the cross section of the instrument. Two circular holes, each 1 inch in diameter, are cut in the centres of two opposite ends of the box; into one of these is fitted a double rotating right and left handed quartz, with its plane of separation parallel to the square faces of the box; over this opening is fitted one end of a tube about 10 inches long, and a Nicol's prism is fitted to the other end.

The instrument is supported by means of a short outer tube, to one end of which is attached a circle graduated to degrees and which is supported steadily on a light tripod stand. An index on the inner tube, and revolving with it about its axis in front of the graduated circle on the outer tube, shows the angle through which the instrument is turned. When the plane of separation of the biquartz is vertical, the index on the tube should be at zero.

To use the polarimeter :

Place the Nicol's prism in the tube with its principal section perpendicular to the plane of separation of the biquartz; place the glass plate so that their planes are perpendicular to the axis of the tube, and the index at zero. Light, entering at the opening at one end of the box, passes perpendicularly through the plates and the biquartz and Nicol's prism. If the light is partially or wholly plane-polarised, turn the tube and box until the plane of separation of the biquartz coincides with the plane of polarisation of the light, the index will then show the inclination of

the plane of polarisation to the vertical. Then turn the axis carrying the glass plates until the light passing through the Nicol's prism is found to be depolarised. The angle through which the plates are turned will give the proportion of plane-polarised light in the beam passing through the instrument, by means of a table of values depending on the angle of incidence of the light.

The mathematical theory of the instrument and a table of values for actual use are given by Prof. Adams in the Philosophical Magazine for March 1871.

CHAPTER III.

CHROMATIC POLARISATION-THE WAVE
THEORY.

WE now proceed to the consideration of the colours produced by plates of crystal when submitted to the action of polarised light. A crystal very commonly used for this purpose is selenite or hydrated sulphate of lime, which is readily split and ground into flat plates of almost any required thickness. If such a plate be placed between the polariser and analyser when crossed, it will be found that there are two positions, at right angles to each other, in which, if the selenite be placed, the field will remain dark as before. The selenite is, in fact, a doubly refracting crystal, and although neither of the rays to which it gives rise follows the ordinary law, yet they may, for sake of distinction, be respectively called the ordinary and extraordinary. The positions in question are those in which the plane of vibration of the ordinary ray coincides with that of the polariser (or analyser), and that of the extraordinary ray with that of the analyser (or polariser). In every other position of the selenite, and notably when it has turned through 45° from either of the positions before mentioned, or neutral