MOUNT WILSON OBSERVATORY. 223 



tion, in the case of nebulae with emission spectra at least, is not a simple 

 reflection. 



The method employed for investigating the relationship in diffuse nebulae 

 between luminosity and associated stars was to plot the photographic 

 magnitude of the stars against the logarithm of the angular extent of the 

 nebulosity expressed in minutes of arc for a given exposure-time. The form 

 of the resulting curve, m+5 log a = constant, indicates that the inverse-square 

 law applies. Within the errors of observation, the constant is that which is 

 derived by spreading the light of a star of magnitude m over a spherical shell 

 of radius a, where Seares's value of 18.8 magnitudes per square second of arc 

 is used as the limiting surface brightness registered in an exposure of one 

 minute. 



This method was applied to a discussion of Curtis's data on planetary 

 nebulae in Publications of the Lick Observatory, volume XIII. A definite 

 relationship was found to exist between m, the magnitude of the central star, 

 and a combination of log e, the shortest relative exposure-time required 

 to register the nebulosity, and log a, the angular distance from the cen- 

 tral star to that portion of the nebula for which e was measured. When a 

 was reduced to ai corresponding to a uniform e and the inverse-square law 

 was assumed, the relationship was found to be m-p-5 log ai = constant, as in 

 the case of the diffuse nebulae. The constant, however, indicates that in the 

 planetaries the photographic surface brightness averages about 4 magnitudes 

 per square second of arc brighter than the photographic light intercepted 

 from the central stars. This discrepancy may possibly find its explanation in 

 the complicated discontinuous character of the nebular spectra and in the 

 extraordinary strength of the ultra-violet continuous spectrum of the central 

 stars. The residuals are rather large, but the indications are that they can 

 be correlated with definite characteristics of the various nebulae. Thus the 

 ring nebulae show uniformly large positive residuals, and the globular types, 

 such as N. G. C. 6572, uniformly large negative residuals. 



Preliminary investigations of the luminosity of non-galactic nebulae have 

 been made by plotting the growth of the nebular images with increasing 

 exposures. For N. G. C. 221, 7619, 7623, and 7625, all of which are globular 

 nebulae with conspicuous nuclei, the curves are represented by the equation 

 2 log a — log E = constant, where a is the diameter and E the exposure-time. 

 This indicates that the inverse-square law of luminosity applies to these 

 nebulae as well as to galactic nebulae. Furthermore, the value of the constant 

 in each of these cases is very nearly that derived on the assumption that the 

 surface brightness at any distance from the nucleus is the luminosity of the 

 nucleus spread over a spherical shell of that particular radius. This line of 

 investigation will be continued in order to cover as wide a field as possible in 

 the non-galactic nebulae. 



Dust Clouds in Space and Dark and Bright Nebula. 



Professor Russell has made a theoretical investigation of the effect of the 

 existence of clouds of dust in interstellar space upon the light of stars and has 

 applied his results to problems connected with the nebulae. His principal 

 conclusions are as follows: 



(1) Of the material which may be supposed to exist in space, by far the most 

 effective in its power to scatter light is pure dust made up of particles of the 



