330 QUARTZ FIBERS. 



not even foel tho intlnenco of a mafifnot. I liavo here a magnet, and on 

 waAiiifj the niajiiiet about near tlic instrument there is no movement of 

 the index at all ; it does not dance up and down the scale, as it cer- 

 tainly would do in the case of a galvanometer, because this magnet 

 would all'eet 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 Professor 

 Langley of Ameri(!a 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 tho one I have just shown, but whether it is or is not 

 as sensitive, it certainly can not compare with this in its freedom from 

 the disturbing etfe(!ts of stray heat falling npon it, or of the magnetic 

 or thermoelectric disturbances which give so mucii trouble where the 

 galvanometer is employed. 



Now this ai)paratns I was recently using in sou)e astronomical ex- 

 periments on the heat of the moon and the stars. As these experi- 

 ments 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 the quartz fiibers, I have thought it well to repeat 

 a typical experiment u])on the moon's heat, but, like Peter Quince, I 

 am in this difticuUy. 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, atid 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 suflicient 

 for the conv^ersation of Pyramus and Thisbe, l)ut that would not do for 

 the purpose of showing the variation of radiation from i)oint to point 

 upon the moon's surface, and as that is the experiment which I now 

 wish to show — an experiment which this instrument enables one to 

 make with the greatest ease and certainty — it is necessary to have 

 something better than a " lanthorn " and a bunch of thorns. Therefore I 

 have been obliged, as tho 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 nuide by taking an egg-shell and 

 painting it white. That egg-shell is now placed uj)on 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 

 concave mirror upon a translucent screen. There is in addition an- 

 other mirror which tlirows a snuiU image of the same moon upon the 

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

 the screen a black si)ot which represents the sensitive surface of the 

 radio-micrometer. That bears the same i>roportion to the moon which 

 you see on the screen as the sensilivc surface of the radio-micrometer 

 bears to the image of the moon that is cast upon it. Now the two mir- 



