the Nicol and the medium which causes polarization. The whole beam is now suffused with color, the tint of which changes, as did the tints on the waves, while the Nicol is turned round. And not only so, but while the Nicol remains at rest, the tints are to be seen scattered in a regular and definite order in different directions about the sides of the beam. This may be shown by reflecting from a looking-glass a side of the beam not visible directly, and by comparing the tint seen by reflection with that seen direct. But this radial distribution of colors may also be shown in a more striking manner, by putting together two half-plates of quartz of the kinds which have the property of distributing the colors in opposite orders, and by observing the result along the line of junction. The compound plate here used is known by the name of a biquartz, and affords one of the most delicate tests of the presence of polarized light. In this case, when the Nicol is turned round, the colors of the two halves follow one another in opposite orders; and as each series is completed twice in a revolution of the Nicol, the halves of the quartz will be of the same color four times in a revolution—twice of one color and twice of its complementary.
The colors which we have here seen are those which would be observed, as before remarked, upon examining a clear sky in a position at right angles to that of the sun: and the exact tint visible will depend upon the position in which we hold the Nicol, as well as upon that of the sun. Suppose, therefore, we direct our apparatus to that part of the sky which is all day long at right angles to the sun, that is, to the region about the north-pole of the heavens (accurately to the north-pole at the vernal and autumnal equinox); then, if on the one hand we turn the Nicol round, say in a direction opposite to that of the sun's motion, the colors will change in a definite order; if, on the other, we hold it fixed, and allow the sun to move round, the colors will change in a similar manner. And thus, in the latter case, we might conclude the position of the sun, or, in other words, the time of day, by the colors so shown. This is the principle of Sir Charles Wheatstone's polar clock; one of the few practical applications which this branch of polarization has yet found. The action of such a clock may be thus roughly shown: There is now projected upon the screen a dial-plate, in which the hours are arranged in their usual order, but are crowded together into half their usual space, viz., twelve hours occupy half instead of the entire circle. The inner part of the disk is covered with a plate of selenite (mica would serve the purpose equally well), which is capable of revolving about its centre, and which, as you see, in a particular position shows color more strongly than in any other. An hour-hand is roughly drawn upon the plate. The apparatus here used is furnished with two Nicol's prisms, the hinder one of which imitates the polarizing effect of the sun, while that in front is the instrument with which we should examine the north-pole of the sky. The whole is now so arranged that when the plate shows
