90 SECTION PHYSICS. 



the results, it would take much more time than we have at our disposal. 

 This is the actual apparatus which was used in most of my experi- 

 ments. I may mention that the reflector of my telescope is three feet 

 diameter. The larger telescope, which is six feet, was not used for the 

 purpose, because it had only a very limited range of motion, only 

 about twenty minutes to half an hour on each side of the meridian, 

 and therefore there was only that length of time on any night on 

 which to make observations, and clearly, anywhere but especially 

 in an uncertain climate like ours, it is desirable to seize on every 

 moment when the moon may be sufficiently high in the sky. The focal 

 length is about twenty-seven feet, and this apparatus was presented to 

 the mirror, being fixed in the mouth of the tube, the small condensing 

 mirrors being fixed in the focus of the large mirror. These mirrors are 

 of about three inches focal length, and the thermopiles are placed in 

 their foci, so that the whole light that enters the tube of the telescope, 

 or falls on to a space of three feet in diameter is first concentrated by 

 the large mirror on to the surface of each reflector, and then by them 

 concentrated further on to the faces of the thermopiles -| inch diameter. 

 The reason why I used two reflectors was to destroy the disturbing 

 effects of the various strata of the atmosphere, the clouds which might 

 be over it and those arising from heat and cold acting unequal 

 on the thermopiles. The effect of using two mirrors is that they 

 neutralize in one another the differences produced by heat and cold 

 falling on the apparatus, as they act in opposite directions. If the 

 heat of the moon be turned alternately on this reflector and on that, 

 you pimply get the effect of the moon doubled, but by throwing it on 

 one and then off it, you get the single effect. Many of the details of 

 the apparatus are hardly of general importance, because it is adapted 

 to the instrument for which it is used. The mounting of the instrument 

 was at that time an altazimuth which, as all astronomers know, is an 

 inconvenient one to use for this purpose, as it is necessary constantly 

 to work both the altitude and azimuth motions in following the 

 motions of the heavenly bodies. The image being seen by the attendant 

 on each mirror alternately, he swings the tube of the telescope which is 

 suspended by a rope until he sees the moon's image on the reflector 

 where the slight dust or tarnish allows it to be seen. There is only one 

 point I think I need allude to further, and that is the construction of 



