334 



NATURE 



{Aug. 22, 1872 



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

 sky, and that on a clear bright day we sweep the Iieavens with 

 our apparatus, or polariscope, as it is called, we shall find traces 

 of polarisation colours 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, 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 intermediate 

 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 

 produces 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 callrd, 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 visilde, as is well 

 known by its reflexion from the motes or floating bodies, in fact 

 by the dust in the air. But if we clear the air ot dust, as I now 

 do by burning it with a spirit lamp placed underneath, the beam 

 disappears from the parts so cleared, and the space becomes 

 dark. If, therefore, the air were absolutely pure and devoid of 

 matter foreign to it, the azure of the sky would be no longer 

 seen, and the heavens would appear black ; the illumination of 

 objects ^^■ould be strong and glaring on the one side, and on the 

 other their shadows would be deep, and unrelieved by the dif- 

 fused light to which we aie accustomed. 



Now, setting aside the dust, of which we may hope that there 

 is but little on the downs behind your town, or out to sea in 

 fron% there are always minute particles of water floating in the 

 atmosphere. These vary in size from the great rain drops 

 which fall to earth on a sullry day, through the intermediate 

 foims of mist and of fine fleecy cloird, to the absolutely invisible 

 minuteness of pure aqueous vapour which is present in the 

 brightest of skies. It is these particles which scatter the solar 

 rays, and suffuse the heavens with light. And it is a curious 

 fact, established by Prof Tyndall while operating with minute 

 traces of gaseous vapours (which I can only notice in passing, 

 because it belongs only in part to our present subject), that while 

 coarse particles scatter rays of every colour equally — in other 

 words, scatter white light — finer particles scatter fewer rays from 

 the red end of the spectrum, while the finest scatter only those 

 from the blue end. And in accordance with this law, clouds are 

 white, clear sky is blue. 



But beside this fact, viz., that light scattered laterally from 

 fine particles is blue, the same philosopher perceived that light 

 so scattered is polarised ; and by that observation he again con- 

 nected the celestial phenomena described above with laboratory 

 experiments. 



By a slight modification of his experiment, due to Prof. Stokes, 

 I hope to make this visible to the audience. It will probably be 

 in your recollection that when polarised light passed through a 

 Nicol, its intensity is unaltered when the Nicol is in one position, 

 but it is destroyed when it is in another at right angles to the 

 first. I now pass the beam from the electric lamp through a 

 tube of water containing a few drops of mastic dissolved in alco- 

 hol. The mixture so formed holds fine particles of mastic in a 

 stale of suspension ; these scatter the light laterally, so as to be 

 visible, I hope, to the entire audience. And if we were to 

 examine with a Nicol this scattered light, we should find the phc- 

 noirrena of polarisation. But, better still, we can cause the light 

 to pass through the Nicol before being scattered, and produce 

 the same effect, not only upon the particular part to which our 

 eye is directed, but upon the whole body of scattered light. As 

 tire Nicol is turned, the light seen laterally begins to fade ; and 

 when the instrument has been turned through a right angle, the 

 only parts remaining visible are those which are reflected from 

 the larger impurities floating in the water independently of the 

 mastic. An effect still more beautiful, and at the same time 



more instructive, can be produced by interposing, as was done 

 in the case of reflexion, a plate of quartz between the Nicol and 

 the medium which causes polarisation. The whole beam is now 

 suffused with colour, 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 I'est, 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 reflexion with that seen direct. 

 But this radial distribution of colours 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 

 colours in opposite onlers, and by observing the result along the 

 line of junction, lire compound plate here used is known by 

 the name of a biquartz, and affords one of the most delicate tests 

 of the presence of polari ed light. In this case, when the Nicol 

 is turned round, the cjlours 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 colour four times in a revolution — twice of one colour and 

 twice of i's complementary. 



The colours 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, theref ire, 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 norih 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 t3 that of the sun's motion, 

 the colours will change in a definite order ; if, on the other, we 

 hold it fi.xed, and allow the sun to move round, the colours 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 colours so shown. This is the principle ot 

 Sir Charles Wlieatstone's polar clock ; one of the few practical 

 applications which this branch of polarisation 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 disc 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 colour 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 polarising 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 brightest colour the hand 

 points to XII., say noon. As the back Nicol is turned round, 

 say as the sun begins to sink, the colour fades ; and when the 

 plate is turned so as to restore the colour, the hand points to I. 

 Similarly, as the back Nicol is turned gradually further, repre- 

 senting the passage of the sun westward during the afternoon, 

 the position of the plate giving the strongest colour, as indicated 

 by the hand, corresponds to the successive hours of the dial ; and 

 when the Nicol his b -en turned through go°, that is, when the 

 sun has reached the horizon, the hand has moved from XII. to 

 VI. In this way, as its inventor has remarked, a dial may be 

 constructed which will work equally well in sunshine or in shade, 

 or even when the sun itself is overcast, provided only that there 

 be a patch of clear sky to the north. 



Up to this point we have reproduced in an experimental fashion 

 the general every-day phenomena, both celestial and terrestrial, 

 which give rise to polarisation ; and we hare given such general 

 account of them as will serve to connect them together, and to 

 show that they all belong to one system of laws affecting the 

 nature of light. I should, however, regret, and I feel confident 

 that you would share in that regret, if we were to leave the sub- 

 ject with its surface as it were merely scratched, and without any 

 attempt to penetrate deeper into its substance. With your per- 

 mission, therefore, we will devote such time as you may be still 

 willing to grant me to a few elementary experiments in polarisa- 

 tion, which, while certainly not less beautiful than those which 

 you have already seen, will, perhaps, better illustrate the nature 

 of the proce ses %\h:c'i we are now trying to investigate. 



