SUNLIGHT, SEA, AND SKY. 55 



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 dis- 

 tributing 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 op- 

 posite 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 w r e 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 de- 

 pend 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 w r e 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 



