SOLAR ECLIPSE EXPEDITIONS— MITCHELL IQl 



taken by different observers at all the eclipses in the past 40 years, 

 both hands would be unnecessary. The arch enemy of the eclipse 

 astronomer, cloudy weather, has foiled many attempts, yet those that 

 escaped the clouds have not obtained perfect results through poor 

 focus or poor seeing or from inaccurate timing of the exposures. 



At a total solar eclipse, the flash spectrum may be observed at the 

 beginning and at the end of totality, either with or without slit, and 

 by the use of prisms or grating. If without slit, the individual lines in 

 the spectrum are a series of arcs of various intensities and of different 

 lengths depending on the heights to which the solar gases extend 

 above the sun's surface. The photographic plate may be fixed for 

 each exposure or it may be moved gradually by an arrangement first 

 tried out successfully by Campbell in 1905. A variation of the fixed 

 plate is the "jumping film" successfully tried in 1932 by the Lick 

 expedition. The principle is the same as with the movie camera, the 

 film being moved between exposures made on a fixed film. The 

 essential differences are that the camera is a spectrograph, the films 

 are much larger in size than the standard 35-mm variety and the 

 exposures and times for changing film may be each about ji second in 

 duration. 



The fixed plate requires very exact timing of the exposures in order 

 to obtain imprints from the most important layers of the cluomosphere, 

 those of lowest levels. The moving plate or the jumping film starts 

 the exposures a half mmute or more before the beginning of totality in 

 order to catch the first fiasli so that the human element is partly 

 eliminated. Each of the three methods has both its own peculiar 

 advantage and at the same time its own disadvantages. Without 

 here going into details, it is evident that on account of the very great 

 difficulty of obtaining successful photographs of the chromospheric 

 spectra aheady alluded to, it will be wise to utilize more than one 

 type at an eclipse. 



Astronomers do not go on an expedition merely to see the phenom- 

 enon and enjoy the beauties of the corona. Frequently the astrono- 

 mer is in a dark room making exposures or he has such a large program 

 to carry through that he can obtain merely a passing glimpse of the 

 corona. Expeditions are to secure photographs which are measured 

 and studied at home far distant from the few thrilling and excited 

 moments of the total eclipse. In the year 1905 in Spain, the author 

 obtained with an exposure of 2 seconds a photograph of the flash 

 spectrum on which he has spent considerably more than 5 years of 

 concentrated work. 



Such a photograph is used to supplement researches being carried 

 out daily on the sun without an eclipse by the use of a powerful 

 spectrograph such as is attached to the 150-foot Mount Wilson tower 



31508—38 12 



