330 QUARTZ FIBERS. 



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 cer- 

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

 would affect a galvanometer at the other end of the room. We liave 

 then a degree of sensibility which is certainlj^ not easily developed in any 

 other way. I must except however the instrument which Professor 

 Laugley 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 can not 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 ex- 

 perimiMits 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 fibers, I have thought it well to repeat 

 a ti'pical experiment upon the moon's lieat, bnt, like Peter Quince, I 

 am in this difi&cuUy. 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"andabuuchof 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 i)oint 

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

 wish to show — an ex[)eriment 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 the moen 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 jilaced 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 

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

 other 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 mir- 



