l6o REPORTS Of* INVESTIGATIONS AND PROJECTS. 



(8) A continuation of the spectrographic study of the solar rotation indi- 

 cates that the rotation rate did not vary appreciably between 1906 and 1908. 

 The observations confirm the earlier ones in showing that different lines 

 give different velocities. The calcium line A 4227 gives a higher rotational 

 velocity and a smaller equatorial acceleration than the general reversing 

 layer. The Ha line of hydrogen gives a still higher rotational velocity and 

 a very small equatorial acceleration. 



(9) The motion of the reversing layer in the vicinity of sun-spots appears 

 to be seriously influenced by the motion in the vortices. 



(10) By a photographic comparison, it has been shown that the light of 

 the Great Nebula in Andromeda and of three star-clusters contains a larger 

 proportion of the less refrangible rays than the light of stars of the same 

 spectral type. On account of the great distance of these objects this is 

 probably due to the selective absorption of light in space. 



(11) Photographs of nebulae, made with the 60-inch reflector, bring out 

 details not previously recorded, notably a remarkable spiral structure near 

 the center of the Andromeda Nebula. 



(12) Photographs of stellar spectra, made with a spectrograph of 18 feet 

 focal length, used in conjunction with the 60-inch reflector, are on a suffi- 

 cient scale to permit the determination of the pressure in the atmosphere of 

 Arcturus. 



(13) In the laboratory a close agreement has been found between the 

 separation into components of lines in sun-spot spectra and their resolution 

 in the magnetic field, both as to magnitude and character of the separation. 



Preston's law, — y == const., holds approximately in the case of iron if mean 

 A 



values of AA, representing a considerable number of lines, are used. Experi- 

 ments have been carried out to show to what extent inclination of the lines 

 of force and polarization by the apparatus may explain certain effects ob- 

 served in spot spectra. 



( 14) Electric- furnace investigations have provided a classification of spec- 

 trum lines on the basis of their readiness of response to temperature excita- 

 tion. The laboratory evidence as to the lower temperature of sun-spots has 

 been extended and the foundations laid for a general classification of light 

 sources according to temperature. 



(15) Spark spectra under pressure have been photographed for com- 

 parison with the spectra of the limb and center of the sun. 



(16) A study has been made of iron lines which are relatively strength- 

 ened by the spark discharge, and a number of lines have been added to 

 this list. 



(17) The fluted spectrum of calcium burning in hydrogen has been meas- 

 ured from large-scale photographs, and the correspondence of these flutings 

 with lines in spot-spectra has been worked out in detail. 



