66 REPORTS ON INVESTIGATIONS AND PROJECTS. 



Finall}', to mention only some of the principal modes of attack, 

 without going further into detail, the bolometer or radiometer fur- 

 nishes the necessary means of measuring the radiation energy of the 

 spots and photosphere in various parts of the spectrum, thus supple- 

 menting in an important way the instruments alread}' mentioned. 

 All of these modes of solar observation have been brought into use on 

 Mount Wilson, thanks to the Snow telescope, which supplies the first 

 requisite — a large and well-defined solar image within a laboratory. 



The photographs of spectra obtained with this instrument well 

 illustrate its advantages over an equatorial telescope. The long and 

 powerful Littrow spectrograph, having a combined collimator and 

 camera lens of i8 feet focal length, could not possibly be attached to 

 such a telescope as the 40-inch Yerkes refractor. The photographs 

 obtained with this spectrograph are naturally far superior to the best 

 we were able to make with the 40-inch, and the precision with which 

 thej' can be measured is in proportion to the respective focal lengths 

 of the spectrographs used in the two cases — 42 inches at Lake Geneva 

 and 216 inches at Mount Wilson. The third-order spectrum of the 

 same grating (the one formerlj' used in the Kenwood spectrohelio- 

 graph) was employed in both cases. A great number of widened 

 lines are shown in the spot spectra and the reversals, of H and K are 

 admirably brought out. Systematic work with this instrument has 

 been going on since the latter part of August and will be continued 

 regularly. 



Certain conclusions, of importance in their bearing on future work, 

 have resulted from the experiments already made. These are (i) the 

 advantage of using a large solar image, in the present case 6.7 inches 

 in diameter, produced by the mirror of 60 feet focal length ; (2) the 

 advantage of very high resolving power and linear dispersion in the 

 spectrograph; (3) the importance of providing for a simultaneous 

 attack on the same solar phenomena with a suitable battery of inde- 

 pendent instruments. I have accordingly designed a coelostat tele- 

 scope of 60 feet focal length and a powerful spectrograph especially 

 for work on the spectra of sun-spots, flocculi, and the chromosphere, 

 and the study of the solar rotation. It is hoped that this instrument, 

 which is to be erected next year, will combine a high degree of efii- 

 cieucy with moderate cost. A special merit of this telescope lies in 

 the fact that it will set free the Snow telescope for uninterrupted 

 work with the spectroheliograph during the early morning hours of 

 fine definition, and at the same time permit the various spectroscopic 

 phenomena to be simultaneously recorded. 



