Mar. 19, 1 8 74 J 



NATURE 



385 



transmit light from the furthest limits of the stellar uni- 

 verse. The ether is however certainly inoperative in the 

 diffusion of light now under consideration. But a very 

 simple experiment will suffice to show that such a diffu- 

 sion 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 an electric lamp or from the sun be allowed to 

 pass through a room its track becomes visible by its 

 reflection from the motes of floating bodies, in fact by 

 the dust in the air. But if the air be cleared of dust 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 no longer be seen and the heavens would appear 

 black ; the illumination of objects would be strong and 

 glaring on one side, and on the other their shadows would 

 be deep and unrelieved by the diffused light to which we 

 are accustomed. Now, setting aside the dust, there are 

 always minute particles of water floating in the atmo- 

 sphere. These vary in size from the great raindrops 

 which fall to earth on a sultry day, through intermediate 

 forms of mist and of fine fleecy cloud, 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 remarkable fact, established by Prof. 

 Tyndall, while operating with minute traces of gase- 

 ous vapours, that while coarser particles scatter rays 

 of every colour, in other words scatter white light, finer 

 particles scatter fewer rays from the red end of the spec- 

 trum, while the finest scatter only those from the blue 

 end. And in accordance with this law clouds are white, 

 clear sky is blue. 



But the point which most concerns us here is the fact, 

 also discovered by Prof Tyndall, that light scattered 

 laterally from fine particles is polarised. The experiment 

 by which this is most readily shown is as follows : Allow 

 a beam of sclar or other strong light to pass through a 

 tube about thirty inches long filled with water, with 

 which a few drops of mastic dissolved in alcohol have 

 been mixed. The fluid so formed holds fine particles of 

 mastic in a state of suspension, which scatter the light 

 laterally ; and if the scattered light be examined with a 

 Nicol traces of polarisation will be detected. But better 

 still, instead of using the scattering particles as a polariser 

 and the Nicol as an analyser, we may polarise the 

 light before it enters the tube and use the particles 

 as an analyser, and thus produce the same effect 

 as before, not only upon the particular point of the 

 beam to which the eye is directed, but upon the whole 

 body of scattered light. As the Nicol is turned the light 

 seen laterally begins to fade ; and when the instrument 

 has been turned so as to cut off all vertical vibrations, the 

 only parts remaining visible in a horizontal direction will 

 be those reflected from the larger impurities floating in 

 the water independently of the mastic. The direction of 

 vibration of the light polarised by lateral scattering is 

 easily remembered by the fact that the vibrations must be 

 perpendicular both to the original and to the scattered 

 beam ; if, therefore, the latter be viewed horizontally, 

 they must be perpendicular to two horizontal straight 

 lines at right angles to one another, i.e. they must be 

 vertical. 



An effect still more beautiful, and at the same time 

 perhaps more instructive, may be produced by interposing 

 a plate of ciuartz between the Nicol and the tube. The 

 whole beam then becomes suffused with colour, the tint 

 of which changes for a given position of the spectator 

 with the argle through which the Nicol is turned. 



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 size of the beam. 

 But this radial distribution of colours may also be shown 



in a more striking manner, by using a bi-quartz, which as 

 explained before distributes the colours in opposite direc- 

 tions. The beam should in every case be viewed at 

 right angles ; the more obUquely it is viewed the less 

 decided is the polarisation. 



The colours here seen are those which would be ob- 

 served upon examining a clear sky in a position 90° from 

 that of the sun ; and the exact tint visible will depend 

 upon the position in which the Nicol is held, as well as 

 upon that of the sun. Suppose, therefore, that a Nicol and 

 quartz plate be directed 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 the Nicol be turned round, say, in a 

 direction opposite to that of the sun's motion, the colours 

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

 remain stationary while the sun moves 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 the day, by the colours so shown. 

 This is the principle of Sir Charles Wheatstone's Polar 

 clock, which is one of the few practical applications which 

 this branch of polarisation has yet found. 



Figs. 18 and 19 represent general forms of this instru- 

 ment described in the following passage by the inventor. 



" At the extremity of a vertical pillar is fixed, within 

 a brass ring, a glass disc, so inclined that its plane is 

 perpendicular to the polar axis of the earth. On the 

 lower half of this disc is a graduated semicircle divided 

 into twelve parts (each of which is again sub-divided 

 into five or ten parts), and a'jainst the divisions the 

 hours of the day are marked, commencing and termi- 

 nating with VI. Within the fixed brass ring, containing 

 the glass dial plate, the broad end of a conical tube is 

 so fitted that it freely moves round its own axis ; this 

 broad end is closed by another glass disc, in the centre of 

 which is a small star or other figure, formed of thin films 

 of selenite, exhibiting when examined with polarised light 

 strongly contrasted colours ; and a hand is painted in 

 such a position as to be a prolongation of one of the 

 principal sections of the crystalline films. At the smaller 

 end of the conical tube a Nicol's prism is fixed so that 

 either of its diagonals shall be 45" from the principal 

 section of the selenite films. The instrument being so 

 fixed that the axis of the conical tube shall coincide with 

 the polar axis of the earth, and the eye of the observer 

 being placed to the Nicol's prism, it will be remarked 

 that the selenite star will in general be richly coloured, 

 but as the tube is turned on its axis the colours will 

 vary in intensity, and in two positions will entirely dis- 

 appear. In one of these positions a smaller circular disc 

 in the centre of the star will be a certain colour (red, for 

 instance), while in the other position it will exhibit the 

 complementary colour. This effect is obtained by placing 

 the principal section of the small central disc 22i° from 

 that of the other fihns of selenite which form the star. 

 The rule to ascertain the time by this instrument is as 

 follows : — the tube must be turned round by the hand of 

 the observer until the colour star entirely disappears 

 while the disc in the centre remains red ; the hand will 

 then point accurately to the hour. The accuracy with 

 which the solar time may be indicated by this means wfll 

 depend on the exactness with which the plane of polarisa- 

 tion can be determined ; one degree ot change in the 

 plane corresponds with four minutes of solar time. 



" The instrument may be furnished vvith a graduated 

 quadrant for the purpose of adapting it to any latitude ; 

 but if it be intended to be fixed in any locality, it may be 

 permanently adjusted to the proper polar elevation and 

 the expense of the graduated quadrant be saved ; a spirit- 

 level will be useful to adjust it accurately. The instrument 

 might be set to its proper azimuth by the sun's shadow at 

 noon, or by means of a declination needle ; but an obser- 



