ASTRONOMY — HALE. 6l 



nature. A loo-inch reflector, vv-ith a focal length of 50 feet, should be 

 capable of photographing an immense number of these objects to excel- 

 lent advantage. In view of the work of Chamberlin and Moulton on the 

 nebular hypothesis, and the theoretical study of spiral nebulae upon which 

 the latter is engaged, such observational evidence as the new instrument 

 may afford should prove of the greatest value. Mr. Hooker's gift is 

 especially opportune, in that it will permit the Solar Observatory to con- 

 tinue, after the completion of our 60-inch reflector, the employment of the 

 skilled opticians who have been trained by Mr. Ritchey for their difficult 

 and delicate work. The question as to the type of mounting to be em- 

 ployed, and the various details of its design, need not be considered until 

 the 60-inch reflector has been thoroughly tested in actual use on Mount 

 Wilson. The completion of the optical work can not be expected before 

 the end of four years, while only a year would be required by the Union 

 Iron Works Company to complete the mounting and dome. The liberal 

 terms of Mr. Hooker's letter of gift specifically free the Carnegie Institu- 

 tion from any obligation whatever in accepting the mirror. The source 

 from which the funds are to be obtained to mount and house it is not 

 known, but it may be anticipated with confidence that a donor can be 

 found when the proper time arrives. I have discussed elsewhere in this 

 report the various questions that naturally suggest themselves as to the 

 probability of obtaining success with so large a reflector. 



The second event to which I wish to allude is the formulation and pre- 

 liminary test of the hypothesis that the characteristic phenomena of the 

 spectra of sun-spots may be accounted for on the assumption that the tem- 

 perature of the metallic vapors within sun-spots is below that of the cor- 

 responding vapors in the sun's reversing layer. The spectra of sun-spots 

 have for many years offered an interesting problem to solar physicists. 

 The fact that some of the lines of certain metals are greatly strengthened 

 in them, while others are weakened or obliterated, has given rise to much 

 speculation as to the probable cause of these phenomena. The strength- 

 ened and weakened lines have appeared to be set apart, in a most capri- 

 cious manner, from the unaffected lines of the elements in question, though 

 certain solar observers have been inclined to regard spot spectra as in- 

 dicating merely an increase in the absorptive phenomena visible at all 

 points upon the solar surface. While it can not be said that our hypoth- 

 esis has advanced beyond the first stage, it has nevertheless been dem- 

 onstrated in our laboratory work that the great majority of lines that 

 are strengthened in sun-spots are also strengthened in the laboratory 

 when the temperature of the vapor which gives rise to them is decreased, 

 while a similarly large proportion of the lines that are weakened in sun- 

 spots are weakened in the laboratory under the same conditions. The 



