240 CARNEGIE INSTITUTION OF WASHINGTON. 



The bolometer is of the special vacuum type described in Volume IV of the 

 Annals of the Astrophysical Observatory of the Smithsonian Institution. 



The spectroscope is of the Littrow type, with a 5-inch concave mirror 

 of 45 cm. focal length to collimate the light and focus the spectrum. The 

 prism is of Jena U. V. crown glass with an angle of 18°. It is silvered on the 

 rear face and so is equivalent to a prism of 36°. 



In the first experiments made in July the spectrometer and bolometer 

 were used at the primary focus of the 100-inch reflector, and the galvanometer 

 and photographic recorder were placed in the clock-room. The disturbances 

 due to electric circuits and air currents proved, however, to be too serious to 

 overcome. Accordingly the apparatus was transferred to the constant- 

 temperature room south of the great pier and the telescope was used in the 

 coude form, the light being brought down through the polar axis. The equiva- 

 lent focal length of the telescope in this form is 250 feet. In order to bring 

 the apparatus to the center of the room, use is made of an 8-inch concave 

 mirror of 1-meter focus placed 6 meters beyond the focus of the telescope. 

 This forms, at a distance of 120 cm., on the slit of the spectrometer, an image, 

 the characteristics of which are very similar to those observed previously 

 at the Newtonian focus. Guiding and focusing are accomplished by adjust- 

 ments of the 8-inch mirror; but little guiding is necessary, because of the satis- 

 factory driving of the 100-inch telescope. 



After numerous difficulties had been overcome, connected mainly with 

 disturbances of the galvanometer, a trial was made of the apparatus on the 

 night of August 18. Successful visual observations were obtained of the 

 intensities of the spectra of Aldebaran, Capella, and Betelgeuse, and, on the 

 following day, of the sun. The distribution of intensities in the normal 

 spectrum of Capella, after correction for the effect of the earth's atmosphere, 

 proves to be nearly identical with that of the sun, a result to be expected from 

 a star of its spectral type. The spectral intensities in Betelgeuse are very 

 different, showing a sharp maximum at 0.83 /x in the infra-red, and departing 

 very widely from those in a black body with a corresponding maximum ordi- 

 nate. The spectrum of Aldebaran shows results intermediate between those 

 for the other two stars. 



Although in these observations the period of the single swing of the galvan- 

 ometer mirror was only 1.56 seconds, and the scale-distance employed but 

 4.7 meters, deflections of 14 mm. were observed in the case of Betelgeuse and 

 over one-half that amount for Aldebaran and Capella. At this degree of 

 sensitiveness, the current in the galvanometer corresponding to a deflection 

 of 1 mm. on the scale is 2.5X10 -11 amperes, and the corresponding rise of 

 temperature of the bolometer, 8X10 -8 degrees centigrade. Deflections of 

 0.5 mm. could trustworthily be observed. It appears certain that an increase 

 of at least ten-fold in useful sensitiveness can be employed, together with 

 photographic registration. This will make it possible to obtain satisfactory 

 spectrobolometric observations of stars to the third and perhaps fourth 



magnitudes. 



THE GREEN AURORAL LINE. 



Lord Rayleigh, Campbell, Slipher, and others have shown that the green 

 line characteristic of the spectrum of the polar light may be found very fre- 

 quently in the spectrum of the night sky, even when no aurora is visible. 



