474 



NATURE 



\_March 14, 1889 



One sees clearly why one gets a musical note : the noise of the 

 ■spark is clue to a sudden heating of the air ; now if the heat is 

 •oscillatory, the sound will be oscillatory too, hut both will be an 

 octave above the electric oscillation, if I may so express it, 

 because two heat- pulses will accompany every complete electric 

 vibration, the heat production being independent ol direction of 



■ current. 



Having thus got the frequency of oscillation down to so 

 manageable a value, the optical analysis of it presents no diffi- 



■ culty : a simple 1 loking-glass waggled in the hand will suffice to 

 spread out the spark into a serrated band, just as pan be done 

 with a singing or a sensitive flame, a band too of very much the 

 same appearance. 



Using an ordinary four-square rotating mirror driven electro- 

 ■magnetically at the rate of some two or three revolutions per 

 second, the band is at the lowest pitch seen to be quite 



• coarely serrated ; and fine serrations can be seen with four 

 ' revolutions per second in even the shrill whistling sparks. 



The only difficulty in seeing these eff"ects is to catch them at 

 the right moment. They are only visible for a minute fraction 

 of a revjintion, though the band may appear drawn out to some 

 length. The further away a spark is from the mirror, the more 

 ■drawn out it is, but also the less chance there is of catching it. 



With a single observer it i^ easy to arrange a contact maker 

 ■on the axle of the mirror which shall bring on the discharge at 

 the right place in the revolution, and the observer may then 

 ■conveniently watch for the image in a telescope or opera-glass, 

 though at the lower pitches nothing of the kind is necessary. 



But to show it to a large audience various plans can be adopted. 

 One is to arrange for several sparks instead of one ; another is 

 to multiply images of a single spark by suitably adjusted re- 

 flectors, which if they are concave will give magnified images ; 

 another is to use several rotating mirrors : and indeed I do use 

 two. one adjusted so as to suit the spectators in the gallery. 



But the best plan that has struck me is to combine an inter- 

 mittent and an oscillatory discharge. Have the circuit in two 

 branches, one of high resistance so as to give intermittences, the 

 ■other of ordinary resistance so as to be oscillatory, and let the 

 mirror analyze every constituent of the intermittent discharge 

 into a serrated band. There will thus be not one spark, but a 

 multitude of successive sparks, close enough together to sound 

 almost like one, separate enough in the rotating mirror to be 

 -visible on all sid s at once. 



But to achieve it one must have great exciting power. In spite 

 of the power of this magnificent VVimshurst machine, it takes 

 ■some time to charge up ou- great Leyden battery, and it is 

 tedious waiting for each spark. A VVimshurst does admirably 

 for a single observer, but for a multitude one want-; an instru- 

 ment which shall charge the battery not once only but many 

 limes over, with overflows between, and all in the twinkling of 

 an eye. 



To get this T must abandon my friend Mr. Wimshurst, and 

 return to Michael Faraday. In front of the table is a gi'eat in- 

 duction coil ; its secondary has the resistance needed to give an 

 intermittent discharge. • The quantity it supplies at a single 

 spark will fill our jars to overflowing several times over. The dis- 

 . -charge circuit and all its circumstances shall remain unchanged. 

 [Excite jars by c lil.] 



Running over the gamut with this coil now used as our exciter 

 instead of the Wimshurst machine — everything else remaining 



■ exactly as it was — you hear the sparks give the same notes as 

 before, but with a slight rattle in addition, indicating intermit- 

 tence as well as alternation. Rotate the mirror, and everyone 

 should sep one or other of the serrated bands of light at nearly 



• every break of the primary current of the coil. [Rotating mirror 

 to analyze sparks.] 



The musical sparks which I have now shown you were 



• obtained by me during a special digression^ which I made 

 while examining the effect of discharging a Leyden jar 

 round heavy glass or bisulphide of carbon. The rotation of 

 the plane of polarization of light by a steady current, or by 

 a magnetic field of any kind properly disposed with respect 

 to the rays of light, is a very familiar one in this place. Perhaps 



• it is known also that it can be done by a Leyden jar current. 

 But I do not think it is ; and the fact seems to me very interest- 



>ing. It is not exactly new — in fact, as things go now it may be 

 almost called old, for it was investigated six or seven years ago 



' Most likely it was a conversation which I had with Sir Wm. Thomson, 

 -at Chrsimis. which caused me to see the interest of gett ng slow oscillations. 

 .My att-iition lias mainly been directed to getting them qui:k. 



by two most highly skilled French experimenters, Messrs. 

 Bichat and Blondlot. 



But it is exceedingly interesting as showing how short a time, 

 how absolutely no time, is needed by heavy glass to throw 

 itself into the suitable rotatory condition. Some observers have 

 thought they had proved that heavy glass requires time to 

 develop the effect, by spinning it between the poles of a magnet 

 and seeing the effect decrease ; but their conclusions cannot be 

 right, for the polarized light follows every oscillation in a dis- 

 charge, the plane of polarization being waved to and fro as oft-n 

 as 70,000 times a second in my own observation. 



Very le.\N persons in the world have seen the effect. In fact, 

 I doubt if anyone had seen it a month ago except Messrs. 

 Bichat and Blondlot. But I hope to make it visible to most 

 persons here, though I hardly hope to make it visible to all. 



Returning to the Wimshurst machine as exci'.er, I pass a 

 discharge round the spiral of wire inclosing this long tube of CS-j, 

 and the analyzing Nicol being turned to darkness, there may be 

 seen a faint — by those close to not so faint, but a very momentary 

 — restoration of light on the screen at every spark. [CS^ tube 

 experiment on screen.] 



Now I. say that this light restoration is also oscillatory. 

 One way of proving this fact is to insert a biquartz betweeen 

 the Nicols. With a steady current it constitutes a sensitive 

 detector of rotation, its sensitive tint turning green on one 

 side and red on the other. But with this oscillatory current 

 a biquartz does absolutely nothing. [Biquartz.] 



That is one proof. Another is that rotating the analyzer either 

 way weakens the extra brightening of the field, and weakens it 

 equally either way. 



But the most convincing proof is to reflect the light coming 

 through the tube upon our rotating mirror, and to look now 

 not at the spark, or not only at the spark, but at the faint band 

 into which the last residue of light coming through polarizer 

 and tube and analyzer is drawn out. [Analyze the light in 

 rotating mirror.] 



At every discharge this faint streak brightens in places into a 

 beaded band : these are the oscillations of the polarized light : 

 and when examined side by side they are as absolutely syn- 

 chronnis with the oscillations of the spark itself as can be 

 perceived. 



Out of a multitude of phenomena connected with the Leyden 

 jar discharge I have selected a few only to present to you here 

 this evening. Many more might have been shown, and great 

 numbers more are not at present adapted for presentation to an 

 audience, being imly visible with difficulty and close to. 



An old and trite subject is seen to have in the light of theory 

 an unexpected charm and brilliancy. So it is with a great 

 number of other old familiar facts at the present time. 



The present is an epoch of astounding activity in physical 

 science. Progress is a thing of months and weeks, almost of 

 days. The long line of isolated ripples of past discovery seem 

 blending into a mighty wave, on the crest of which one begins 

 to discern some oncoming magnificent generalization. The 

 suspense is becoming feverish, at times almost painful. One 

 feels like a boy who has been long strumming on the silent key- 

 board of a deserted organ, into the chest of which an unseen 

 power begins to blow a vivifying breath. Astonished, he now 

 finds that the touch of a finger elicits a responsive note, and he 

 hesitates, half delighted, half affrighted, lest he be deafened by 

 the chords which it would seem he can now summon forth 

 almost at will. 



ON THE LIMIT OF THE SOLAR SPECTRUM, 

 THE BLUE OF THE SKY, AND THE 

 FLUORESCENCE OF OZONE. 



nTHIiRE are t»vo facts of particular interest which have been 

 -*- observed in connection with the light which we receive from 

 the sun and the sky. First, though the ultra-violet spectrum of 

 the sun is very well represented by the iron spectrum taken from 

 the electric arc, yet its lengih is nothing like so great, and there is 

 no fading away of feeble lines and a weakening of strong ones, 

 which would be the case if the rays «ere aff>;cted by a turbid 

 medium through which they were transmitted, but there is a 

 sudden and sharp extinction which points to a very definite ab- 

 sorption. Secondly, light from the clearest sky, unaffected by 

 aqueous vapour, is of a deejj and beautiful blue colour, more of 



