To this we may reply that light that is reflected from any body is generally partially polarized in the plane of reflection, and that if we find that the light received from any body is partially polarized in a given plane, we may conclude that it has very likely been reflected in that plane.
Hence then in the case of any celestial body the origin of the light of which is doubtful, the polariscope tells us whether the light is intrinsic or reflected.
It tells us more than this, it tells us the plane in which the reflection has taken place. As the polarization takes place, when it does take place, at the celestial body, all we have to do is to attach an analyzer to the telescope.
A careful application of the above principles has shown that the light from the sun’s corona is partially polarized, and in the same plane as it would be if reflected from small particles in the neighbourhood of the sun: so also a portion of the light of Coggia’s Comet was found to be polarized, and therefore we say that it reflected sunlight in addition to its own proper light.
In what has been hitherto said we have only considered the use of a Nicol, or glass plates, or crystal of Iceland spar as an analyzer, and by the variation of brightness the presence and plane of polarization have been determined; but unless the polarization is somewhat decided, it could not be detected by this method. Advantage is therefore taken of the fact that a plate of quartz rotates the plane of polarization of a ray passing through it, and it rotates the more refrangible colours more than the others, and some crystals rotate the plane one way, and others in the opposite direction: the crystals are therefore called respectively right- and left-handed quartz; the thicker the quartz the greater the angle through which the plane of polarization is twisted.
This supplies us with a most delicate apparatus, which we next describe. A crystal of right- and a crystal of left-handed quartz are taken and cut to such thickness that a ray of any colour, say green, has its plane turned through 90° on passing through each of them. They are then cut into the form of a semicircle and placed side by side. Any change of the angle of polarization will now affect each plate differently. In one plate the colours will change from red to violet, in the other from violet to red.
If now a ray of polarized light, say vibrating in a vertical plane, falls on them, the green rays will have their plane of vibration turned through 90° by each crystal, and the vibration of the green from both crystals will then be in the horizontal plane. Nicol’s prism interposed between the quartz plates and the eye, so as to allow horizontal vibrations to pass, will show the green from both crystals of equal intensity; the rays of other colours, being turned through a greater or less angle than 90°, will not be vibrating horizontally, and will therefore only partially pass through, so green will be the prevailing colour. If now the plane of vibration of the original ray be turned a little out of the vertical, the ray, on the red side of the green, will appear in one half, and that on the violet side of the green in the other: so that immediately the plane of polarization changes, the plates transmit a different colour, and the apparatus must be twisted round through just the same angle as the polarized ray in order to get the crystals of the same colour. It is therefore obvious that the angle made by a polarized ray with a fixed plane is easily ascertained in this manner.
There is also another instrument for detecting polarization which is perhaps more commonly used than the biquartz: it is generally called Savart’s analyser, and is extremely sensitive in its action. On looking through it at any object emitting ordinary light, the white circle of light limited by the aperture of the instrument only is seen; but if any polarized light should happen to be present, a number of parallel bands, each shaded from red to violet, make their appearance; on rotating the instrument a point is found when a very slight motion causes the bands to vanish and others to appear in the intermediate spaces, and knowing the position required for the change of bands with light polarized in a known plane, say the vertical plane, it is easy to find how far the plane of polarization of any ray is from the vertical, by the number of degrees through which the instrument must be turned to change the bands. The construction of the instrument, and especially its action, is not easy to understand without a considerable knowledge of optics, but it may be stated that a plate of quartz is cut, in a direction inclined at 45° to its axis, into two parts of the same thickness; one part is then turned through a right angle and placed with the same surfaces in contact as before; these are fixed in the instrument so that the light shall traverse them perpendicularly to the plane of section; the light then passes through a Nicol’s prism as an analyser to the eye. The lines observed, “black centred” in one position, and “white centred” in the position at right angles to this, are always in the direction before referred to. The delicacy of the test supplied by this arrangement increases as this direction is more nearly parallel or perpendicular to the plane of polarization of the ray under examination.
CHAPTER XXXI.
CELESTIAL PHOTOGRAPHY.—THE WAYS AND MEANS.
We come now last of all to that branch of the work of the physical astronomer which bids fair in the future to replace all existing methods of observation.