1883.] on the Size of Atoms. 205 



optically transparent ; but gradually you will see an exquisite blue 

 cloud. That is Tyndall's "blue sky." You see it now. I take a 

 Nicol's prism, and by looking through it I find the azure light coming 

 from the vapours in any direction perpendicular to the exciting beam 

 of light to be very completely polarised in the plane through my eye 

 and the exciting beam. It consists of light-vibrations in one definite 

 direction, and that, as finally demonstrated by Professor Stokes, it 

 seems to me beyond all doubt, through reasoning on this phenomenon 

 of polarisation,* which he had observed in various experimental ar- 

 rangements giving minute solid or liquid particles scattered through 

 a transparent medium, must be the direction perpendicular to the 

 plane of polarisation. 



What you are now about to see, and what I tell you I have seen 

 through the Nicol's prism, is due to what I may call secondary or 

 derived waves of light diverging from very minute liquid sj^herules, 

 condensed in consequence of the chemical decomposing influence 

 exerted by the beam of light on the matter in the tube, which was all 

 gaseous when the light was first admitted. 



To understand these derived waves, first you must regard them as 

 due to motion of the ether round each spherule ; the siDherule beinc 

 almost absolutely fixed, because its density is enormously greater than 



* Extract from Professor Stokes's paper " On the Change of Eefrangibility of 

 Light," read before the Royal Society, May 27th, 1852, and publislied in the 

 'Transactions' for that date: — 



" § 183. Now this result appears to me to have no remote bearing on the 

 question of the directions of the vibration in polarised light. So long as the sus- 

 pended particles are large compared with the waves of light, reflection takes place 

 as it would from a portion of the surface of a large solid immersed in the fluid, 

 and no conclusion can be drawn either way. But if the diameters of the particles 

 be small compared with the length of a wave of light, it seems plain that the 

 vibrations in a reflected ray cannot be perpendicular to the vibrations in the inci- 

 dent ray. Let us suppose for the present, that in the case of the beams actually 

 observed, the suspended particles were small compared with the lengtli of a wave 

 of light. Observation showed that the reflected ray was polarised. Now all the 

 appearances presented by a plane polarised ray are symmetrical with respect to 

 the plane of polarisation. Hence we have two directions to choose between for 

 the direction of the vibrations in the reflected ray, namely, that of the incident 

 ray, and a direction perpendicular to both the incident and the reflected rays. 

 The former would be necessarily perpendicular to the directions of vibration in 

 the incident ray, and therefore we are obliged to choose the latter, and conse- 

 quently to suppose that the vibrations of plane polarised light are perpendicular 

 to the plane of polarisation, since experiment shows that the plam^ of polarisation 

 of the reflected ray is the plane ot reflection. According to this theory, if we 

 resolve the vibrations in the [horizontal] incident ray horizontally and vertically, 

 the resolved parts will correspond to the two rays, polarised respectively in and 

 perpendicularly to the plane of reflection, into which the incident ray may bo 

 conceived to be divided, and of these the firmer alone is capable of furnishing 



a ray reflected vertically upwards [to be seen by an eye above the line of 



the incident ray, and looking vertically downwards]. And, in fact, observation 

 shows that, in order to quench the dispersed beam, it is suificient, instead of 

 analysing the reflected light, to polarise the incident light in a plane perpendicular 

 to the plane of reflection." 



