SUNLIGHT, SEA, AND SKY. 53 



for the one piu'pose, we may conclude that it will serve equally well 

 for the other. 



And now a word about that simpler apparatus. When light falls 

 upon a transparent substance, part is reflected, part transmitted. If, 

 therefore, the reflected part is polarized (and you have already seen 

 that this is sometimes the case), it is not surprising that the transmit- 

 ted part should be so also. And further, if the polarization by a single 

 reflection or transmission is incomplete, it will become more and more 

 complete by a repetition of the processes. This being so, if we take 

 a pile of glass plates say half a dozen, more or less, the thinner the 

 better and hold them obliquely before our eye at an angle of about 

 30 (say one-third of a right angle) to the direction in which we are 

 looking, we shall have all that is necessary to detect the presence of 

 polarization ; and if, further, we hold a piece of talc or mica, such as 

 is commonly used as a cover to the globes of gas-burners, beyond the 

 pile of plates, color will be produced in the same general manner as 

 with the quartz, although with some essential difference in detail. 



Suppose that we now turn our attention from the sea to the sky, 

 and that on a clear, bright day we sweep the heavens with our appara- 

 tus, or polariscope, as it is called, we shall find traces of polarization 

 colors brought out in a great many directions. But if we observe 

 more closely we shall find that the most marked effects are produced 

 in directions at right angles to that of the sun, when, in fact, we are 

 looking across the direction of the solar beams. Thus 5 if the sun 

 were just rising in the east or setting in the west, the line of most 

 vivid effect would lie on a circle traced over the heavens from north to 

 south. If the sun were in the zenith, or immediately overhead, the 

 most vivid effects would be found round the horizon ; while at inter- 

 mediate hours the circle would shift round at the same rate as the 

 clock, so as always to retain its direction at right angles to that of the 

 sun. 



Now, what is it that can produce this effect or what even pro- 

 duces the light from all parts of a clear sky? The firmament is not a 

 solid sphere or canopy, as was once supposed ; it is clear, pure space, 

 with no contents, save a few miles of the atmosphere of our earth, 

 and beyond that the impalpable fluid or ether, as it is called, which is 

 supposed to pervade all space, and to transmit light from the further 

 limits of the stellar universe. But, apart from this ether, which is 

 certainly inoperative to produce the sky appearance as we see it, a 

 very simple experiment will suffice to show that a diffusion, or, as it 

 has been better called, a scattering of light, is due to the presence of 

 small particles in the air. If a beam from the electric lamp, or from 

 the sun if we had it, be allowed to pass the room, its track becomes 

 visible, as is well known by its reflection from the motes or floating 

 bodies, in fact by the dust in the air. But if we clear the air of dust, 

 as I now do by burning it with a spirit-lamp placed underneath, the 



