March 13, 1884] 



NATURE 



463 



occur, to give us the s^ound of a musical nute. If the barometer 

 varies once a minute you would not perceive that as a musical 

 note. But suppose by any mechanical action in the air, you 

 could cause the barometric pressure — the air pressure — to vary 

 much more rapidly. That change of pressure which tlie baro- 

 meter is not quick enough to show to the eye, the ear hears as a 

 musical sound if the period recurs twenty times per second. If 

 it recurs twenty, thirty, forty, or fifty times per second, you 

 hear a low note. If the period is gradually accelerated, you hear 

 the low note gradually rising, becoming higher and higher, more 

 and more acute, and if it gels up to 256 periods per second, we 

 have a certain note called C in the ordinary musical notation. I 

 believe I describe it correctly as the low note C, of the tenor 

 voice — the gravest C that can be made by a flute. The note of 

 a two-foot organ pipe open at both ends has 256 periods per 

 second. Go on higher and higher to 512 periods per second, 

 and you have the C above that — the chief C of th= soprano 

 voice. Go above that to 1024, you get an octave higher. You 

 get an octave higher always by doubling the number of vibra- 

 tions per second, and if you go on till you get up to about 5000 

 or 6000 or 10,000 periods per second, the note becomes so shrill 

 that it ceases to excite the human ear and you do not hear it any 

 longer. The highest note that can be perceived by the human 

 ear seems to be something like 10,000 periods per second. I 

 say "something like," because there is no very definite limit. 

 Some ears cease to hear a note becoming shriller and shriller 

 before other ears cease to hear it ; and therefore I can only say 

 in a very general way, that something like lo,coo periods per 

 second, is about the shrillest note the human ear is adapted to 

 hear. We may define musical notes, therefore, as changes of 

 pressure of the air, regularly alternating in periods which lie 

 Isetween 20 and 10,000 per second. 



Well now, are there vibrations of thirty or forty or fifty or a 

 hundred thousand or a million of periods per second in air, in elastic 

 solids, or in any matter affecting our sense? We have no 

 evidence of the existence in matter of vibrations of very much 

 greater frequency than 10,000 or 20,000 or 30,000 per second, 

 but we have no reason to deny the possibility of such vibrations 

 existing, and having a large function to perform in nature. But 

 when we get to some degree of frequency that I cannot put 

 figures upon, to something that may be measured in millions, if 

 not in hundred-thousands of vibrations per second, we have not 

 merely passed the limits of the human ear to hear, but we have 

 passed the limits of matter, as known to us, to vibrate. Vibra- 

 tions transmitted as w aves through steel, or air, or water, cannot 

 be more frequent than a certain number, which I cannot now put 

 a figure to, but which, I say, may be reckoned in hundred- 

 thou-ands or a few millions per second. 



But now let us think of light. Light we know to be an 

 influence on the retina of the eye, and through the retina on the 

 optic nerve ; an influence dependent on vibrations whose fre- 

 quency is something between 400 million millions per second 

 and too million millions per second. Now we have a vast gap 

 between 400 per second, the sound of a rather high tenor voice, 

 and 400 million millions per second, the number of vibrations 

 corresponding to dull red light — the gravest red light of the 

 prismatic spectrum. Take the middle of the spectrum — yellow 

 light — the period of the vibrations there is in round numbers 

 500 million millions per second. In violet light we have 800 

 million millions per second. Beyond that we have something 

 that the eye scarcely perceives — does not perceive at all perhaps 

 — but which I believe it does perceive, though not vividly : we 

 have the ultra-violet rays, known to us chiefly by their photo- 

 graphic effect, but known also by many other wonderful experi- 

 ments, that within the last thirty years have enlarged rur know- 

 ledge of light to a most marvellous degree. We have invisible 

 rays of light made visible by letting them fall on a certain kind 

 of glass, glass tinged with uranium — that yellowish green glass, 

 sometimes called canary glass or chameleon glass. Uranium 

 glass has a property rendering visible to us invisible rays. You 

 may hold a piece of uranium glass in your hand, illuminated by 

 this electric light, or by a candle, or by gas light, or hold it in 

 the prismatic spectrum of while light, and you see it glowing 

 accoiding to the colour of the light which falls upon it ; but 

 place it in the spectrum beyond the visible violet end, where 

 w ithout it you see nothing, w here a piece of chalk held up seems 

 quite dark, and the uranium glass glows with a mysterious altered 

 colour of a beautiful tint, revealing the presence of invisible 

 rays, by converting them into rays of lower period, and so 

 rendering them visible to the eye. The discovery of this 



property of uranium glass was made by Prof. Stokes, and the 

 name of fluorescence from fluor spar, which he found to have the 

 same property, was given to it. It has since been discovered 

 that fluorescence and phosphorescence are continuous, being 

 extremes of the same phenomenon. I suppose most persons 

 here present kiiow the luminous paint made from sulphides of 

 calcium and other materials, which, after being steeped in light 

 for a certain time, keep on for hours giving out light in the 

 darkness. Persistence in emission of light after the removal of 

 the source, w'hich is the characteristic of those phosphorescent 

 objects, is manifested also, as Edmund Becquerel has proved, 

 by the uranium glass, and thus Stokes' discovery of fluorescence 

 conies to be continuous \\ ilh the old known phenomenon of 

 phosphorescence, to which attention seems to have been first 

 called scientifically by Robert Boyle about 2co years ago. 



There are other rays, that we do not perceive in any of these 

 ways, but that we do perceive by our sense of heat : heat rays as 

 they are commonly called. But in truth all rays that we call 

 light have heating effect. Radiant heat and light are one and indi- 

 visible. 'I here are not two things, radiant heat and light ; radiant 

 heat is identical with li jht. Take ablack hot kettle into a dark 

 room, and look at it. You do not see it. Hold your face or your 

 hand near it, and you perceive it by what Bunyan would have called 

 Feel Gate ; only now we apply the word feeling to other senses 

 as well as Touch. You perceive it before you touch it. You 

 perceive it with the back of your hand, or the front of your 

 hand ; you perceive it w ith your face, yes, and with your eye, 

 but you do not see it. Well, now, must I justify the assertion 

 that it is not light ? You say it is not light, and it is not so to 

 you, if you do not see it. There has been a good deal of logic- 

 chopping about the words here ; we seem to define in a vicious 

 circle. We may begin by defining light — " It is light if you see 

 it as light ; it is not light if you do not see it." To save circum- 

 locution, we shall take things in that way. Radiant heat is 

 light if we see it, it is not light if we do not see it. It is not 

 that there are two things ; it is that radiant heat has differences 

 of quality. There are qualities of radiant heat that we can see, 

 and if we see them we call them light ; there are qualities of 

 radiant heat we cannot see, and if we cainiot see them we do 

 not call them light, but still call them radiant heat : and that on 

 the whole seems to me to be the best logic for this subject. 



By the bye, I don't see Logic among the studies of the 

 Birmingham and Midland Institute. Logic is to language and 

 grammar what mathematics is fo common sense; logic is 

 etherealised grammar. I hope the advanced student in grammar 

 and Latin and Greek, who needs logic perhaps as much as, per- 

 haps more than, most students of science and modern languages, 

 will advance to logic, and consider logic as the science of using 

 words, to lead him to know exactly what he means by thera 

 when he uses them. More ships have been wrecked through 

 bad logic than by bad seamanship. When the captain writes 

 down in his log — I don't mean a pun here, log has nothing to do 

 with logic — the ship's place is so-and-so, he means that it is the 

 most probable position — the position which, according to pre- 

 vious observations, he thinks is the most probable. After that, 

 supposing no sights of sun or stars or land to be had, careful 

 observation of spe d and direction shows, by a simple reckoning 

 (called technically the dead reckoning), where the ship is next 

 day. But sailors too often forget that what they put down in 

 the log was not the ship's place, but what to their then know- 

 ledge w as the most probable position of the ship, and they keep 

 running on as if it was the true position. They forget the 

 meaning of the very words in which they have made their entry 

 in the log, and through that bad logic more ships have been run 

 on the rocks than by any other carelessness or bad seamanship. 

 It is bad logic that leads to trusting to the dead reckoning, in 

 running a course at sea ; and it is that bad logic which is the 

 cause of those terribly frequent wrecks ; of steamers, otherwise 

 well conducted, in cloudy but perfectly fine weather, running on 

 rocks at llie end of a long voyage. To enable you to understand 

 precisely the meaning of your result when you make a note_of 

 anything about your own experience or experiments, and to 

 understand precisely the meaning of what you write down, is 

 the province of logic. To arrange your record in such a manner 

 that if you look at it afterwards it will tell you what it is worth, 

 and neiiher more nor less, is practical logic ; and if you exer- 

 cise that practical logic, you will find benefits that are too 

 obvious if you only think of any scientific or practical subject 

 with which you are familiar. 



There is danger then of a bad use of words, and hence of bad 



