. The Astrophysical journal. before the instrument could be relied upon forconsistent readings with the degree of accuracy sought, the essentialfeatures in the design of the apparatus in its final form will bementioned. In regard to the thermo-element, it was seen at once that asurface-element would be necessary, of small heat-capacity, henceshort period, and of the maximum sensitiveness. After futileefforts to obtain a suitable element in the market, the requirementswere met by a thermopile specially designed and constructed forthe purpose by W. W. Coblentz, of the Bureau of Standards,Washing
. The Astrophysical journal. before the instrument could be relied upon forconsistent readings with the degree of accuracy sought, the essentialfeatures in the design of the apparatus in its final form will bementioned. In regard to the thermo-element, it was seen at once that asurface-element would be necessary, of small heat-capacity, henceshort period, and of the maximum sensitiveness. After futileefforts to obtain a suitable element in the market, the requirementswere met by a thermopile specially designed and constructed forthe purpose by W. W. Coblentz, of the Bureau of Standards,Washington. The element is a bismuth-silver combination of sym-metrical design, having a circular receiving surface 5 mm in diam-eter, and a resistance of ohms. Tests reveal a sensitiveness For a detailed description of the construction of bismuth-silver thermo-couples,see Instruments and Methods Used in Radiometry, by W. W. Coblentz, Bulletinof the Bureau of Slaiidards, 9, 15, 1912. THERMOPILE IN PHOTOGRAPHIC PHOTOMETRY 257 2^. Fig. I 258 IIARLAX TRIE STETSON of one microwatt per o. 18 microvolt, and a period of but afew seconds duration. The element was incased in a brass recep-tacle and the whole mounted in the interior of a box of noncon-ducting material packed with cotton wool, the opening to the wellthrough wliich the radiation passes being closed with a thin glassplate. Such insulation has proved wholly satisfactory under allordinarv conditions of temperature. With the seclusion of thethermopile, it became necessary to introduce a small mirror at Mto reflect light from the element into a telescope of short focus at E,which serves to locate the stellar image upon the receiver. Any error due to imperfect centering of the stellar image uponthe thermopile is avoided by using the finding eyepiece and makingseveral settings for each star. The agreement is generally betteramong the image-readings than among the plate-readings, as thelatter are distributed more or less uniformly ab
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