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 conditions on different nights.
The radiometers used in these measurements were minute thermocouples 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
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-holder in the camera which is part of the equipment of the telescope. By removing the screws S, S, it was therefore 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.
Referring to Fig. 1, it may be added that the star light after reflection 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 thermojunction 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.
