6o6 



NA TURE 



[October i6, 1890 



avoidable shaking during the day. The point to which I 

 specially wish to refer is this. In electroscopes and all electro- 

 static apparatus one puts in a dish of sulphuric acid, which is 

 an abomination, in order to keep the atmosphere dry. I have 

 in this electroscope such a dish, but it is filled with water in 

 order to keep the atmosphere moist. Experiments carefully 

 made, using the same box — everything the same — except that 

 in one case the insulating stem was made of quartz, and in the 

 second case it was made of the best flint glass, well washed, of 

 the same shape and size, show that, if the atmosphere is per- 

 fectly dry, the electricity escapes from both at the same rate ; 

 but that, if the atmosphere is perfectly moist, the electricity 

 escapes from the leaves insulated by the clean-washed flint glass 

 only too quickly ; whereas, from the leaves insulated by the 

 quartz, the rate is identically the same as it was in either case 

 when the atmosphere was perfectly dry. 



I have said that these fibres are uniform in diameter, and fine 

 and smooth and strong, and that they glisten with all the 

 colours of the spider web, but that they are far more brilliant. 

 It was naturally rather a curious point to note what a spider 

 ■would do if by any chance she should find herself on such a web, 

 and now that I am dealing with live and wild animals which 

 cannot possibly be trained the conditions are such as to render 

 the success of an experiment entirely a matter of chance. How- 

 ever, I propose to make use of the spider as a test of the 

 very great smoothness and slipperiness of one of these fibres. 

 There are here three little spiders which have been good enough, 

 since they came to Leeds, to spin upon these little wooden 

 frames their perfect and beautiful geometrical webs. I have 

 succeeded in placing one of these frames in the lantern without 

 disturbing the spider, which you can now see waiting upon her 

 web. I must now, without disturbing the peace of mind of 

 the spider, carry her to a web of quartz ; and therefore [it is 

 necessary that the spider should be fortunate enough to catch a 

 fly. Now, instead of bringing a fly I will make an ordinary 

 tuning-fork buzz against the web. She immediately pounces 

 upon the imaginary fly, and thus I can without frightening her 

 place her upon the quartz fibre. Unfortunately this spider has 

 slipped and has got away, but with another I am more suc- 

 cessful. I intended to show that the small and common garden 

 spider could not climb the quartz fibre, but for some reason this 

 spider is able to get up with difficulty ; however I shall not spend 

 any more time upon this experiment. 



I shall now at once speak about the instrument which actually 

 led me to the invention of the process for making quartz fibres. 

 This, which I have called a radio-micrometer, is an instru- 

 ment of very great delicacy for measuring radiant heat from 

 such a thing as a candle, a fire, the sun, or anything else which 

 radiates heat through space. 



The radio-micrometer which I wish to show this evening 

 is resting upon a solid and steady beam, and as usual its index is 

 a spot of light upon the scale. Vou see that that spot of light 

 is almost perfectly steady. Now the heat that I propose 

 to measure, or rather the influence of which I intend to show 

 you, is the heat which is being radiated from a candle fixed in 

 the front of the upper gallery some 70 or 80 feet from 

 the instrument ; and in order that you may be sure that the in- 

 dication of the instrument is due to the heat from the candle, 

 and not to any manipulation of the apparatus on the beam, I shall 

 perform the experiment as follows. None of the apparatus at this 

 end of the room will be touched or moved in any way ; but by a 

 string I shall simply pull the candle along a slide up to a stop, 

 at which position it will shine upon the sensitive part of the 

 radio-micrometer. Instantly the spot of light darts along the 

 scale for a distance of ten feet, and then after leaving the scale 

 it comes to rest upon the face of the balcony five or six seconds 

 after it began to move. Now if the candle is allowed to move 

 back through about a foot, you will see that the instrument will 

 cool down at once — it is at present suffering from the heat which 

 falls upon it from the distant candle ; but it will cool down at 

 once, and the index will go back to its old place. It is very 

 nearly at its old place now. I will now let the candle shine 

 upon it again. The index at once goes on to the balcony as 

 before, and now that the candle is moved away again, the index 

 has assumed its old place upon the scale. 



That really shows that we have here the means of measuring 

 heat with a degree of delicacy, and also with a degree of cer- 

 tainty, ease, and quickness, which has never yet been equalled. 

 It is probable that the measure which I have given of the degree 

 of delicacy that I have reached in my astronomical apparatus — 

 namely, that the heat of a candle more than two miles away can 

 NO. 1094, VOL. 42] 



certainly be felt — will not seem so absurd now that you have seen 

 this less perfect apparatus at work, as it does to people whose 

 experience is limited by the thermopile or their senses. 



You can now see the spot of light ; it is perfectly quiet in its 

 old place. I wish to show you that this instrument is unlike 

 those which are ordinarily used for this purpose. All the heat, 

 the very considerable heat, due to this electric arc lamp, 

 is actually falling on the instrument, but not upon its sensi- 

 tive surface, and there is no indication. There are a large 

 number of people in the room — it does not feel the heat 

 from them. Stray heat which it is not meant to feel — which 

 is not in the line along which it can see, or feel — has no influence 

 upon it. When the candle was moved to the place to which it 

 was looking, it felt the heat, and you saw the movement of the 

 index. What is perhaps more important than all is, that it is an 

 instrument which does not even feel the influence of a magnet. 

 I have here a magnet, and on waving the magnet about near the 

 instrument there is no movement of the index at all ; it does not 

 dance up and down the scale, as it certainly would do in the case of 

 a galvanometer, because this magnet would affect a galvanometer 

 at the other end of the room. We have then a degree of 

 sensibility which is certainly not easily developed in any other 

 way. I must except, however, the instrument which Prof. 

 Langley of America has recently brought to a great state of 

 perfection. I am unable to state, from want of information, 

 whether his instrument is as sensitive as the one I have just 

 shown, but whether it is or is not as sensitive it certainly cannot 

 compare with this in its freedom from the disturbing effects of 

 stray heat falling upon it, or of the magnetic or thermo-electric 

 disturbances which give so much trouble where the galvanometer 

 is employed. 



Now this apparatus I was recently using in some astronomical 

 experiments on the heat of the moon and the stars. As these 

 experiments could only be made with an instrument such as 

 this, possessing extreme sensibility and freedom from extraneous 

 disturbances, and as this instrument is both the cause of the 

 discovery and the first result of the application of quartz fibres, I 

 have thought it well to repeat a typical experiment upon the 

 moon's heat, but, like Peter Quince, I am in this difficulty. As he 

 said, "There is two hard things, that is to bring the moonlight 

 into a chamber." In fact, at the present time the moon has not 

 risen, and if it had we should not be much better off". Peter 

 Quince proposed that they should in case of moonlight failing 

 have a lanthorn and a bunch of thorns. That no doubt was 

 sufficient for the conversation of Pyramus and Thisbe, but that 

 would not do for the purpose of showing the variation of radiation 

 from point to point upon the moon's surface, and as that is the ex- 

 periment which I now wish to show — an experiment which this in- 

 strument enables one to make with the greatest ease and certainty 

 — it is necessary to have something better than a lanthorn and a 

 bunch of thorn'!. Therefore I have been obliged, as the moon is 

 not available, to bring a moon. Now this moon is a real moon ; 

 it is not a representation ; it is not a slide ; it is a real moon, and 

 it is made by taking an egg-shell and painting it white. That egg- 

 shell is now placed upon a stand, and is illuminated by the sun — 

 that is, an electric light — and in order that the moon may be 

 visible the room must be darkened. The moon is now shining 

 in the sky. An image of the moon is cast by means of a con- 

 cave mirror upon a translucent screen. There is in addition 

 another mirror which throws a small image of the same moon 

 upon the radio-micrometer. There is one more thing to explain. 

 There is upon the screen a black spot which represents the 

 sensitive surface of the radio-micrometer. That bears the same 

 proportion to the moon which you see on the screen as the 

 sensitive surface of the radio-micrometer bears to the image of 

 the moon that is cast upon it. Now the two mirrors are 

 arranged to move by clockwork, so as to make the two images 

 travel at proportional rates. The moon is travelling with the 

 dark edge foremost, and now that the terminator of the moon 

 has come upon the sensitive surface, the heat is felt and the 

 deflection of the instrument is the result. Now as the moon is 

 gradually travelling through the sky, the radiation is slowly 

 and steadily increasing, because the radiation from the moon 

 gets greater and greater, as the point at which the sun is shining 

 vertically— that is, a point at right angles with the terminator— is 

 approached ; it is here a maximum, and then it falls back, and as 

 soon as the moon has gone off" the instrument, you will see the 

 index fall back almost suddenly. But there is something more. 

 This moon in one respect is better than the other moon. At 

 the present time it represents the moon nineteen days old, a moon, 

 that is to say, which is waning, and which goes through the sky 



