June 28, 1883] 



NA TURE 



205 



ticular tint of colour. The wave-length you see, in the 

 distance from crest to crest of the waves travelling up the 

 long model when I commence giving a simple harmonic 

 oscillation to the lowest bar. I have here a convex lens 

 of very long focus, and a piece of plate glass with its back 

 blackened. When I press the piece of glass against the 

 glass blackened behind, I see coloured rings ; the pheno- 

 menon will be shown to you on the screen by means of 

 the electric light reflected from the space of air between 

 the two pieces of glass. This phenomenon was first ob- 

 served by Sir Isaac Newton, and was first explained by 

 the undulatory theory of light. [Newton's rings are now 

 shown on the screen before you by reflected electric 

 light.] If I press the glasses together, you see a dark 

 spot in the centre ; the rings appear round it, and there 

 is a dark centre with irregularities. Pressure is required 

 to produce that spot. Why ? The answer generally 

 given is, because glass repels glass at a distance of two or 

 three wave-lengths of light ; say at a distance of 1/5,000 

 of a centimetre. I do not believe that for a moment. 

 The seeming repulsion comes from shreds or particles of 

 dust between them. The black spot in the centre is a 

 place where the distance between them is less than a 

 quarter of a wave-length. Now the wave-length for yellow 

 light is about 1/17,000 of a centimetre. The quarter of 

 1/17,000 is about 1/70,000. The place where you see the 

 middle of that black circle corresponds to air at a distance 

 of less than 1/70,000 of a centimetre. Passing from this 

 black spot to the first ring of maximum light, add half a 

 wave-length to the distance, and we can tell what the 

 distance between the two pieces of glass is at this place ; 

 add another half wave-length, and we come to the next 

 maximum of light again ; but the colour prevents us 

 speaking very definitely because we have a number of 

 different wave-lengths concerned. I will simplify that 

 by reducing it all to one colour, red, by interposing a red 

 glass. You have now one colour, but much less light 

 altogether, because this glass only lets through homo- 

 geneous red light, or not much besides. Now look at 

 what you see on the screen, and you have unmistakable 

 evidence of fulcrums of dust between the glass surfaces. 

 When I put on the screw, I whiten the central black 

 spot by causing the elastic glass to pivot, as it were, 

 round the innumerable little fulcrums constituted by the 

 molecules of dust ; and the pieces of glass are pressed 

 not against one another, but against these fulcrums. 

 There are innumerable — say thousands - of little particles 

 of dust jammed between the glass, some of them of per- 

 haps 1/3,000 of a centimetre in diameter, say 5 or 6 wave- 

 lengths. If you lay one piece of glass on another, you 

 think you are pressing glass on glass, but it is nothing of 

 the kind ; it is glass on dust. This is a very beautiful 

 phenomenon, and my first object in showing this experi- 

 ment was simply because it gives us a linear measure 

 bringing us down at once to 1/100,000 of a centimetre. 



Now I am just going to enter a very little into detail 

 regarding the reasons that those four lines of argument 

 give us for assigning a limit to the smallness of the mole- 

 cules of matter. I shall take contact electricity first, and 

 very briefly. If I take these two pieces of zinc and copper 

 and touch them together at the two corners, they become 

 electrified, and attract one another with a perfectly defi- 

 nite force, of which the magnitude is ascertained from 

 absolute measurements in connection with the well-esta- 

 blished doctrine of contact electricity. I do not feel it, 

 because the force is very small. You may do the thing 

 in a measured way ; you may place a little metallic knob 

 or projection on one of them of 1/100,000 of a centimetre, 

 and lean the other against it. Let there be three such 

 little metal feet put on the copper ; let me touch the zinc 

 plate with one of them, and turn it gradually down till it 

 comes to touch the other two. In this position, with an 

 air-space of 1/100,000 of a centimetre between them, 

 there will be positive and negative electricity on the zinc 



and copper surfaces respectively, of such quantities as to 

 cause a mutual attraction amounting to 2 grammes weight 

 per square centimetre. The amount of work done by the 

 electric attraction upon the plates while they are being 

 allowed to approach one another with metallic connection 

 between them at the corner first touched, till they come 

 to the distance of 1/100,000 of a centimetre, is 

 2/100,000 of a centimetre-gramme, supposing the area 

 of each plate to be one square centimetre. 



(To be continued.) 



DEATH OF THE PRESIDENT OF THE 

 ROYAL SOCIETY 



I T is with the profoundest regret that we an- 

 *■ nounce the death of Mr. Spottiswoode, the 

 President of the Royal Society, at 11.15 yesterday 

 morning. The bulletin issued on Tuesday to the 

 effect that although there was no hemorrhage, still 

 that there was no improvement in Mr. Spottis- 

 woode's condition, boded ill because those who 

 knew him best feared that a reserve of strength, 

 which might perhaps have made way against the 

 further progress of the fever through its later 

 stages, was wanting. 



As the sad news reaches us just as we are going 

 to press, and as indeed we so recently entered at 

 some considerable length into the lifework of him 

 who is now no more, there is no necessity for us 

 on the present occasion to do more than make the 

 above announcement. This, however, must be 

 said : that there is hardly a man of science in 

 this country, and there are very many in other 

 countries, who will not feel that they have lost a true 

 friend, and one of whose friendship any man might 

 have been proud. There is little doubt too that if 

 he had been more sparing of himself in the various 

 duties which were incumbent upon him as Presi- 

 dent of the Royal Society, if he had not so freely 

 given all his thoughts and all his exertions to any 

 scientific question which was going on, there might 

 have been more time for relaxation, and there might 

 have been strength to have tided over the illness 

 which has now laid him low. 



NOTES 

 We regret to have to announce the death of General Sir 

 Edward Sabine, K.C.B., which occurred on the 26th inst. at 

 Richmond, where he had been residing for the last twelve 

 months. He was in his ninety-fifth year, having been born 

 October 14, 17S8. 



At the meeting of the Paris Academy of Sciences on Monday 

 last week the following message concerning the eclipse observa- 

 tions from M. Janssen, dated San Francisco, was read : — 

 " Janssen : discovery of the Fraunhofer spectrum and the dark 

 lines of the solar spectrum in the corona, showing cosmical 

 matter around the sun. Large photographs of the corona and 

 the circumsolar regions to a distance of 15 , in search for intra- 

 Mercurial planets. Palisa and Tronvdot : Exploration of the 

 circumsolar regions; no intra-Mercurial planets found. Trouve- 

 lol : Sketch of the corona. Tacchini : Polarisation of the corona 

 and streamers ; spectrum of the streamers, showing analogy 



