MOUNT WIIvSON SOLAR OBSERVATORY. 177 



THE ONE-HUNDRED-INCH TELESCOPE. 



The details of the design of the lOO-inch telescope have made excellent 

 progress during the year under the immediate direction of Mr. Ritchey. The 

 experience gained during the last year with the 6o-inch reflecting telescope 

 has been of the greatest value in guiding the design of the larger instrument. 



As stated in the last report, the glass disk, weighing 4.5 tons, which was 

 secured from the St. Gobain firm, contained so many flaws that it seemed 

 inadvisable to accept it. Since this disk was made the St. Gobain firm has 

 been continuing the attempt to secure a perfect casting. A very large fur- 

 nace and melting-pot capable of holding 20 tons of material has been con- 

 structed and improved methods of annealing have been introduced. In the 

 spring of this year a successful disk was cast, but unfortunately, owing to 

 defects in the mold, strains were introduced in the process of annealing and 

 the disk was broken. With the experience gained from these trials we may 

 reasonably hope for a perfect disk before very long. 



The large grinding-machine for the 100-inch disk has been completed dur- 

 ing the year and the shaping and rough-grinding of the 60-inch plane mirror 

 to be used in testing the larger disk has been nearly finished. 



THE INTERNATIONAL SOLAR UNION MEETING. 



The fourth Conference of the International Union for Cooperation in Solar 

 Research was held on Mount Wilson during the week of August 29 to Sep- 

 tember 3, 1910. About 80 members of the Union and invited guests were 

 present. A brief outline of the program of the Conference follows : 



August 29. Inspection of the Pasadena offices, instrument and optical shops, and 



physical laboratory. 

 August 30. Journey to Mount Wilson. 

 August 31. (i) Opening address by Professor Hale. 



(2) Report of the executive committee. 



(3) Report of the committee on standards of wave-length. 

 At 8 p. m. Address by Mr. Abbot. 



September i. Report of the committee on the measurement of solar radiation. 



Report of the committee on the investigation of the spectra of sun-spots. 

 Report of the committee for the organization of eclipse observations. 

 At 8 p. m. Address by Professor Kapteyn. 

 September 2. 



Morning session : 



Report of the committee on the determination of the solar rotation by 



means of the displacements of lines. 

 Report of the committee on work with the spectroheliograph. 

 Afternoon session : 



Discussion of a proposal to extend the scope of the Union. 

 Nomination and election of committees. 



Meetings of the astrographic chart committee and of the committee on the 

 Kapteyn Selected Areas were held in connection with the Conference. 



Important action was taken by the Solar Union on a number of questions 

 connected with solar and stellar research. In particular, we may refer to the 

 adoption of secondary standards of wave-length ; the recommendation of 

 certain instruments as standards for the measurement of radiation ; the prog- 

 ress made toward additional cooperative work among sun-spot observers and 



12 — YB 



