MEASURING FIE AT FROM STARS 435 



• 



tory at Mt. Hamilton, California. The reflecting mirror is three feet 

 in diameter. The altitude of the station is a little over 4,000 feet. 

 The summer months being rainless; there being no fog or dew; the 

 night temperature being only a few degrees lower than the day time — 

 these were items which made it possible to have fairly uniform condi- 

 tions on different nights. 



The radiometers used in these measurements were minute thermo- 

 couples with receivers 0.3 to 0.4 millimeter in diameter; i. e., about 

 as large as the punctuation mark at the end of this sentence. These 

 thermocouples, the elements of which were bismuth and platinum, were 

 mounted in glass receptacles, as shown in Fig. 1, from which the air 

 could be evacuated. The vacuum- was then maintained by occasionally 

 heating metallic calcium, Ca, contained in a quartz-glass tube shown in 

 Fig. 1. Metallic calcium has the property of absorbing atmospheric 



Fig. 1. Showing the Glass Receptacle which Contains the Theemocouples, E_, 

 AND THE Calcium Ca Used to Maintain a Vacuum. 



gases when warmed to a low red heat. This glass receptacle was then 

 mounted in a brass box as shown in Fig. 2, which was made especially to 

 take the place of the plate-hoilder in the camera which is part of the 

 equipment of the telescope. By removing the screws S, S, it was there- 

 fore a matter of only a few minutes to dismount this radiometric outfit 

 and substitute the plate-holder. In this manner part of the night was 

 spent in making radiometric measurements on stars, after which the 

 telescope was surrendered to another observer who was photographing a 

 newly discovered satellite of Jupiter. 



Eeferring to Fig. 1, it may be added that the star light after re- 

 flection from the telescope mirror passes through a fluorite window, F, 

 and is brought to focus upon the receiver, E, of the thermocouple 

 where the rays are absorbed thus heating the thermojunction. This 

 extremely minute amount of heat is sufficient to warm the thermo- 

 junction a few hundred-thousandths of a degree and thus generate an 

 electric current which passes through the coils of a miniature tangent 

 galvanometer, shown in Fig. 3. "Unfortunately one sees nothing of 

 these coils of wire which are imbedded in two blocks of Swedish iron. 



