214 CARNEGIE INSTITUTION OF WASHINGTON. 



The Mount Wilson photographic map of the sun-spot spectrum, on a 

 scale of 1 centimeter to the angstrom, has been completed by Mr. Eller- 

 man for the region X3900-X6600. This scale, supplemented by the use 

 of a Nicol prism and compound quarter-wave plate over the slit of 

 the 75-foot spectrograph of the 150-foot tower telescope, suffices to sepa- 

 rate the components and to show the polarization phenomena of many 

 lines affected by the magnetic field. The numerous band lines and the 

 changes of line intensity due to the reduced temperature in spots are 

 also well shown. The map should prove of service in solar and labor- 

 atory investigations and in the study of red stars under high disper- 

 sion (p. 224). 



The infra-red region of the solar spectrum, rendered accessible by the 

 use of plates sensitized with dicyanin, has been photographed by Mr. 

 Brackett as far as X9900. The great majority of the 550 lines measured 

 are of terrestrial origin, but through the detection of their displacements 

 produced by the solar rotation, Mr. Brackett has identified about 50 

 solar lines (p. 229). 



Our investigations of nebulae have been considerably extended, and 

 now embrace a wide variety of objects. Mr. Pease and Mr. Duncan 

 have continued to photograph nebulae of interest with the 60-inch and 

 100-inch telescopes, with special reference to proper motions and the 

 problem of rotation in spirals (p. 232). 



Provisional measures by Mr. van Maanen of 32 points in the spiral 

 nebula M 33 indicate the presence of internal motions analogous to 

 those previously found in the case of M 101, namely, an outward mo- 

 tion along the arms of the spiral. The number of points for which this 

 general result does not hold is so small that it may be accepted as the 

 characteristic feature of the motion. The photographs compared were 

 both taken with the 60-inch reflector, one by Mr. Ritchey 10 years ago, 

 the other by Mr. Duncan in August. The mean motion of the neb- 

 ular points for the 10-years' interval, relative to the comparison stars, 

 is about 0''2, which is of the same order as that found in the case 

 of M 101. 



Mr. Hubble has made a study of the three known variable nebulae, 

 one of which (N. G. C. 6729) has shown decided changes within a single 

 day. These remarkable objects do not appear to give indications of 

 actual motion ; from the available evidence their variation seems to be 

 the result of irregular brightening and obscuration of a nebulous back- 

 ground. Mr. Hubble has also taken numerous long-exposure photo- 

 graphs with the large field of the 10-inch photographic refractor and a 

 camera of 6 inches focus for the purpose of determining the distribution 

 of the dark markings on the sky. The results suggest that they lie on a 

 plane inclined at a small angle with the galactic plane, and coinciding 

 with that of the local cluster of B stars (pp. 233-235). 



