MOUNT WILSON SOLAR OBSERVATORY. 247 



INVESTIGATIONS IN PROGRESS. 



SOLAR RESEARCH. 



INSTRUMENTS. 



The principal changes made during the year in the equipment for 

 solar research include the provision of a 6-inch Warner & Swasey equa- 

 torial refractor, with an Evershed spectroscope by Hilger, to }3e used 

 mainly in finding eruptions and other changing phenomena for photo- 

 graphic observation with the Snow and tower telescopes ; the addition 

 of an outer tower and dome, and the improvement of the spectrograph 

 and spectroheliograph of the 60-foot tower telescope; and the con- 

 struction of the Koch registering micro-photometer, now nearly com- 

 pleted, which will also be used in connection with the stellar and 

 laboratory work. 



The 60-foot tower telescope was originally built as cheaply as possible, 

 in view of the fact that so radical a departure fro i existing designs 

 might fail to justify itself in practice. Its manifest advantages over 

 the Snow telescope, both in definition of the solar image and perform- 

 ance of the underground spectrograph, soon led to the construction of 

 the 150-foot tower telescope, which is still more efficient. It then 

 became desirable_^to provide an outer tower, dome, and vertical tube 

 for the 60-foot tower telescope, as the coelostat and other instruments 

 were insufficiently protected from the weather. The opportunity was 

 also utilized to make some needed improvements in the quick and slow 

 motions of the coelostat and to provide the spectrograph with a Fabry- 

 Perot interferometer, for use in Mr. St. John's investigations on the 

 pressure and motion of the gases of the solar atmosphere. The tem- 

 porary building at the foot of the tower has been replaced by a concrete 

 structure, enlarged sufficiently to contain equipment needed in vacuum 

 work with both arc and spark, including a device for producing simul- 

 taneous exposures on the solar and comparison spectra. With these 

 improvements, which will make the 60-foot tower telescope completely 

 fire-proof, it is expected that this instrument can be used for many 

 classes of work, thus leaving the 150-foot tower telescope free for 

 studies of the general magnetic field and the various other researches 

 which demand a large solar image and the highest possible dispersion. 

 As the new spot cycle opens, the pressure upon the time of the three 

 solar telescopes is becoming so great that such relief will be very 

 advantageous. 



The almost unprecedented rains of last winter forced the walls of 

 the 75-foot spectrograph well, in spite of their heav>^ covering of water- 

 proofing material. This led to serious dampness near the bottom, which 

 threatened to interrupt the work. Fortunately the difficulty has been 

 removed by the installation of an electrically driven exhaust fan, which 

 removes the air from the well, and causes a downward flow from with- 



