QUARTZ FIBERS. 329 



which I have called a radio-micrometer, is an iustrnineut of very great 

 delicacy for measuring radiant heat from such a thing as a caudle, 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. You see that that spot of light is almost perfectly 

 steady. Now the heat that I projuise to measure, or rather the influence 

 of which 1 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 indication 

 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 experi- 

 ment as follows. None of the apparatus at this end of the room will 

 be touched or moved in any way ; but by a string 1 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 caudle 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 certainty, 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 can- 

 dle more than two miles away can 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 consider- 

 able heat, due to this electric arc lamp, is actually falling on the in- 

 strument, but not upon its sensitive 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 per- 

 haps more important than all is that it is an instrument which does 



