194 CARNEGIE INSTITUTION OF WASHINGTON. 



in the case of exposures extending over several hours. Some excellent 

 negatives of this character, however, have been obtained with the 100-inch 

 telescope. 



Humason has also photographed several planetary nebulae and a number of 

 miscellaneous objects for purposes of classification. 



STUDIES OF LUMINOSITY IN NEBULA. 



The last annual report summarized the evidence found by Hubble, which 

 indicates that galactic nebulosity derives its luminosity from the radiation 

 of stars associated with it. In diffuse nebulae, which have either absorption 

 or emission spectra, there is a close quantitative agreement between the 

 amount of light received by the nebulosity from the stars and that emitted 

 by the nebulosity. This relationship does not hold in the case of the plane- 

 tary nebulae, the amount of light emitted being considerably greater than 

 that received from the central stars. The discrepancy suggests that the 

 agreement in the case of diffuse nebulae with bright-line spectra is purely 

 accidental, since the associated stars, in them as in planetaries, have spectra 

 which are predominantly continuous or dark-line in character. The phe- 

 nomenon, therefore, can not be one of simple reflection of star light. These 

 conclusions are based upon photographic results from ordinary plates, and 

 studies made in the ultra-violet and red portions of the spectrum may assist 

 in the interpretation of the nature of the absorbed and emitted radiation. 



Hubble has extended his investigations to individual non-galactic nebulae, 

 measuring the densities of a series of exposures made on each plate with the 

 Koch recording microphotometer. The work has been confined almost 

 wholly to elliptical nebulae and the amorphous nuclear regions of a few 

 spirals. Elliptical nebulae are the wholly amorphous objects ranging from 

 globular to spindle in form, in which the brightness seems to decrease steadily 

 outward from the nucleus. The principal characteristic is rotational sym- 

 metry about a dominating semi-stellar nucleus. 



Measurements of the density curves along the diameters of about 25 

 nebulae show that the results can be represented closely by the formula 



D = Do-21og 



e«) 



where D is the density of the photographic image at a distance r from the 

 center, Do is the density at the center, and a is a constant for a particular 

 curve, depending upon the nebula and the diameter along which the curve 

 is constructed. A mean curve may be formed by using r/a as the independent 

 variable and shifting the individual curves along the D-axis into coincidence. 

 The mean residuals over the well-determined portion of this curve are less 

 than 1 per cent. Five curves made from the photovisual images of three 

 nebulae show the same mean slope as the corresponding curves made from 

 photographic images, a result which suggests that in these nebulae, at least, 

 scattering is not a conspicuous phenomenon. 



Since D is the logarithm of the intensity, the distribution of illumination 

 in the images of these nebulae is represented by the formula 



/o 



/ = 



M 



