MOUNT WILSON SOLAR OBSERVATORY. 231 



graphs of promising nebulse, made with special reference to the rigorous 

 requirements of such work, will be taken with the 60-inch and 100-inch 

 reflectors (p. 244). 



The spiral nebula N. G. C. 4594, which we see from a point lying 

 nearly in its own plane, affords an excellent opportunity to study the 

 rotational motion by spectroscopic observations. The high tangential 

 velocity of 335 km. per second for a point 120" from the center, and 

 the fact that the velocity is directly proportional to this distance, are 

 both of peculiar interest. If we may assume that the dimensions and 

 velocities are of the same order of magnitude in this nebula and in 

 M 101, we can at once determine the order of the parallax of N. G. C. 

 4594. The resulting value of 0''0002 indicates the probable remote- 

 ness of this spiral (p. 254). 



The great number of the spiral nebulse, and their striking regularity 

 of form, render them of exceptional interest in the evolutional problem. 

 Any evidence obtainable as to their mode of formation or their degree 

 of condensation at various distances from the center will be eagerly 

 sought. Thus the marked differences in color, revealed by a compari- 

 son of photographs taken with and without color screens, will repay 

 much study (p. 248). The denser nuclear region, giving a spectrum 

 similar to that of a star, sometimes as far advanced as the sun, is 

 relatively yellow, while the spiral arms and the condensations along 

 them are intensely blue. It remains to be seen whether this difference 

 is due to the gaseous character of the outer parts, but a spectrographic 

 observation of a bright knot 10" from the nucleus of the spiral nebula 

 M 33 shows the gaseous nebular spectrum, and thus supports this 

 view (p. 255). 



The color screen, in its various fonns, finds many applications in 

 astronomy. Thus Jupiter and Saturn show new belts and markings 

 when photographed by ultra-violet light. The extension of this 

 method to other planets is Ukely to prove advantageous (p. 243). 



The interpretation of astrophysical observations depends largely 

 upon laboratory experiments, which have gone forward effectively 

 during the year. The electric furnace continues to serve as an 

 efficient means of varying and controlling physical and chemical con- 

 ditions. With its aid the study of the changes of metallic spectra 

 with temperature has gone forward, band spectra have been produced 

 and photographed, and various special experiments have been made 

 (p. 257). These include an examination of the characteristics of the 

 red lithium line, which appears to be present in sun-spots but absent 

 from the general solar spectrum (p. 259), and some work on anomalous 

 dispersion, undertaken to see whether neighboring lines can actually 

 be displaced by mutual influence. The negative results obtained are 

 in harmony with the indications of solar observations (p. 241) as well 

 as theory. Metallic spectra, produced within vacuiun tubes by bom- 



