CAMPBELL— CROCKER ECLIPSE EXPEDITION. 247 



The search for changes in coronal structure evidently remains 

 one of the most interesting eclipse problems of the future. The 

 photographs secured at points as widely separated in time as possible 

 should be made with instruments of long focus, forty feet or more, 

 and under conditions as nearly identical as practicable. Slow plates, 

 such as Seed 23, preferably of the non-halation type, or suitably 

 backed to avoid internal reflections, should be used, and the program 

 of exposures at the different stations should be of identical effi- 

 ciency. The utmost pains should be taken to have the plates in 

 perfect focus and the diurnal motion accurately allowed for. The 

 camera should be so designed that heated air within the camera 

 itself would not detract from the definition. There remains the 

 factor beyond control, the different states of the atmosphere at the 

 various stations, and this is a serious factor indeed, as will be recog- 

 nized by every observer who has endeavored to compare measures 

 of sharply defined photographs with others of the same object but 

 less definite. 



Copies of the photographs obtained in 1918 by the Lick Observa- 

 tory have been distributed to other institutions known to be inter- 

 ested in the subject, and if a comparison of these with photographs 

 obtained at other stations should reveal definite evidence of coronal 

 changes in the intervals we shall be very pleased indeed. 



Polarized Light in the Corona. 



A study of the polarized light in the corona has long been recog- 

 nized as of great importance. Much remains to be done in this 

 field of inquiry. The photography of the corona through double 

 image prisms (slide No. 15) has both advantages and disadvan- 

 tages. The latter arise in part from the factor of chromatic aber- 

 ration when utilizing coronal rays having a great range of wave- 

 length values. The definition under these conditions is unavoidably 

 not all that should be desired, and some uncertainty in the quanti- 

 tative results necessarily follows. The method of inclined plane 

 glass reflectors in front of the coronal cameras, as used successfully 

 by Dr. Perrine (sHde No. 16), has its advantages. In this slide the 

 richness of polarized light in the corona is indicated by the greater 

 vertical dimensions of the right hand image, and the greater hori- 

 zontal dimensions of the left-hand image. 



