MOUNT WILSON OBSERVATORY. 203 



considerations indicate that the diameter of more than 30 stars brighter than 

 the fourth magnitude can be measured with this instrument, the construction 

 of which has been undertaken in our instrument and optical shops (p. 241). 



In the department of stellar photometry reference may be made to the 

 progress in the determination of the photographic magnitudes of the stars in 

 the Selected Areas under the direction of Mr. Scares. The combination of the 

 Mount Wilson and Groningen magnitudes is now complete for 67 areas and 

 half finished for 72 others. The study of the colors of the stars in the areas 

 of the 30° zone is well advanced and similar observations have been made 

 upon a few other fields of exceptional interest. In one of these, Selected Area 

 No. 110, which lies between two branches of the Milky Way south of Cygnus, 

 the star density is remarkably low and the stars are red, no color-index of less 

 than 0.5 mag. having been observed among the stars between the fourteenth 

 and nineteenth magnitudes (p. 226). 



A photometric catalogue of the central stars in the planetary nebulae is 

 being made by Hubble, The photographic magnitudes of 20 stars and the 

 photovisual of 10 have been determined so far, with color-indices ranging 

 from —0.2 to —0.7 mag. 



Scares and Humason have carried out a further investigation of the 

 magnitude scales for the stars of the Polar Sequence from plates taken with the 

 10-inch refractor. These results, together with the investigation by Jones at 

 Greenwich, appear to leave little doubt of the reliability of the scales now 

 available. Scares has devoted much time to this subject in connection with the 

 preparation of the report of the Commission on Photometry for the meeting 

 of the International Astronomical Union at Rome. A series of standard 

 magnitudes was adopted at this meeting which is based upon eight separate 

 determinations of the photographic scale made at six different observatories. 

 For stars brighter than the sixteenth magnitude the average deviation of a 

 final magnitude for any one observatory is ±0.024 mag. (p. 227). 



The measurement of the total radiation of stars is a subject of great interest 

 and promise, as remarked in a previous report. Nicholson and Pettit, who 

 began the development of special thermo-couples for solar investigations and 

 for use with the Koch microphotometer, have also obtained valuable results 

 in the measurement of stellar radiation. Coblentz made excellent observa.tions 

 of stellar radiation in 1914 with a thermopile attached to the 36-inch Crossley 

 reflector, and discovered that "red stars emit two to three times as much 

 total radiation as blue stars of the same photometric magnitude." One star 

 of class N was found by him to give twice the deflection of a class M star of the 

 same visual magnitude. Nicholson and Pettit find the following values of the 

 ratio of total energy of stars of different spectral types to that of an AO star of 

 the same visual magnitude: AO, 1; K, 3; Ma, 9; Mc, 16; N, 16; Md, 1300 (at 

 minimum). The extraordinarily great proportion of infra-red radiation in the 

 case of the Md stars indicates, as Milne has pointed out, the very low tempera- 

 ture of 1700° C. A very important feature of Nicholson's and Pettit's work is 

 its extreme precision, rendered possible, among other means, by a photo- 

 graphic recording device. Six observations of Vega at different altitudes gave 

 values which, when plotted against air-path, indicated 0.002 magnitude as the 

 largest residual. The application of this method to the detection of very 



