MOUNT WILSON SOLAR OBSERVATORY. 249 



transmitting the violet, blue, yellow, and red regions of the spectrum 

 have been tried. The most useful, because of the moderate exposures 

 required, are the ordinary plate and the isochromatic plate and yellow- 

 filter combination used in determining photovisual magnitudes. With 

 these it has been found, for example, that the central nuclei of the 

 spiral nebulae Messier 51, 94, and 99 are relatively yellow, while the 

 branches of the spirals, and especially the knots and star-like condensa- 

 tions scattered along them, are intensely blue. Other spirals suggest a 

 similar result, although completely satisfactory photographs have not 

 yet been obtained. It is not unlikely that the phenomenon is charac- 

 teristic of this class of objects. 



The behavior of the condensations along the branches is reminiscent 

 of the great photographic activity of the central star in the ring nebula 

 in Lyra. Photographs of this latter object reproduce various results 

 of other observers, such, for example, as variations in the size and struc- 

 ture of the ring for light of different wave-lengths. The color-index 

 of the central star seems to be of the order of —1.5 mag.; but the 

 second star is less active photographically, while the two objects in the 

 edges of the ring, just visible on ordinary plates, are strongly yellow. 



In contrast to the spirals and the ring nebula, the bright planetary 

 N. G. C. 3242 shows no important differences in blue and yellow light. 



Magnitudes and Colors in Clusters. 



The study by Mr. Shapley of the magnitudes in clusters has been 

 extended to several new stellar systems. The investigations are near- 

 ing completion for the open cluster Messier 37 and for the more con- 

 densed group Messier 11, which is situated in one of the dense star- 

 clouds of the southern Milky Way. The magnitudes in the globular 

 clusters Messier 3, 5, 14, and 15 have been partially determined and 

 measures on the fainter stars of the bright double cluster in Perseus 

 have been started. 



Catalogues of the magnitudes and colors in Messier 13 and Messier 

 67, mentioned in last year's report, and an extended discussion of the 

 results are now in press. The earlier results, indicating the absence of 

 light-scattering in interstellar space and the existence of giant red 

 stars in globular clusters, have been confirmed by the investigations of 

 the past years. 



Variable Stars. 



A number of miscellaneous investigations by Mr. Shapley relative 

 to variable stars are under way or have been completed during the year. 

 The provisional determination of the color-indices of the short-period 

 Cepheid variables in Messier 3 indicates that in spectral type and 

 variation of color they are closely similar to the isolated ''cluster-type" 

 variables. The similar change of color with light variation is under 

 detailed consideration for XZ Cygni, period 11 hoiu-s, and a mean 



