260 CARNEGIE INSTITUTION OF WASHINGTON. 



less precise than photovisual. The color curve of the lens is so steep 

 that, when focused for H-y (best focus for faintest images), the ultra- 

 violet light is out of focus and produces diffuse images for all but the 

 faintest stars. A potassium-chromate filter, prepared by Mr. Ander- 

 son, finally solved the difficulty by cutting off the light above X4150. 

 Sharp images are now obtained, entirely comparable with those given 

 by a visual color filter, over a range of at least five magnitudes. The 

 color equation for photographic magnitudes is considerably increased, 

 but this is more than offset by the greater accuracy with which the 

 images can be measured. Another advantage of the filter is that, when 

 used with a Seed process plate, bright-line nebulosity is much reduced 

 in intensity, and involved stars can be measured undisturbed by a 

 nebulous background. Thus the trapezium stars in Orion give well- 

 separated images, free from nebulosity and easily measurable, in spite 

 of the short focus of 45 inches and the large focal ratio F 4.5. 



The systematic observation of colors by Mr. Seares has centered 

 mainly on nebulous stars, galactic clouds, and special fields in some of 

 the obscured regions of the sky for the purpose of obtaining evidence 

 bearing on certain investigations undertaken by him jointly with Mr. 

 Hubble. The program of nebulous stars, with a few exceptions, is 

 complete. The results are described on page 255. The galactic clouds 

 and obscured areas are discussed below. 



A series of the color comparisons of the Selected Areas in the +30° 

 zone with the North Pole is half finished. The results will serve a 

 threefold purpose: provide secondary standards of color, useful for 

 the determination of the plate constants for exposure-ratio photo- 

 graphs of fields south of the zenith, permit a study of color as a func- 

 tion of galactic latitude, and afford a valuable control oa the zero- 

 point determinations of the photographic and photovisual scales es- 

 tablished for the Selected Areas. Much work has also been done on a 

 standardization of the relations between exposure ratio, color-index, 

 and spectral type. 



Most observations hitherto made by the method of exposure ratios 

 have been of individual and rather bright stars. For such objects it 

 is easy to arrange the observations so that gradation differences in the 

 photographic plate for blue and yellow light are eliminated. When 

 bright and faint stars are observed simultaneously, however, the prob- 

 lem is much more difficult. The matter has been given much atten- 

 tion, and results seem to justify the belief that the method of observing 

 and reduction now used is free from this objection. 



Mr. Humason has made photographs of M 22 in a search for variable 

 stars, and of the Andromeda nebula for the detection of possible 

 novae. He has also taken about 100 plates of special fields, many of 

 which have been compared with the pole. 



