124 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1940 



are fairly well represented by taking two pictures per minute. Upon 

 projection, a series of pictures taken at 2 per minute will show mo- 

 tions on the screen which are 720 times faster than in nature. The 

 McMath-Hulbert routine procedure for average objects is to speed 

 up their motions only 360 times. In the pictures shown on the 

 screen the compression factor, or rate of speed-up of the motions, 

 varies from 64 to 360. 



It is only because of the great distance to the sun that we need to 

 increase the apparent motions of prominences on the screen by means 

 of time-lapse photography. In terms of terrestrial experience the 

 actual speeds of solar prominence features are enormous. The 

 slowest observable motions are of the order of 1 km. per second, 

 about the same speed as a bullet from a high-powered rifle. The 

 highest observed speed of a prominence feature is about 750 km. per 

 second. Speeds of 10 or 15 km. per second are very common and 

 may be taken as average values. 



The accompanying photographs, selected from motion pictures, 

 show, as well as is possible in still pictures, some of the behavior of 

 prominence material. The most outstanding feature of prominence 

 motion, the overwhelming preponderance of motion toward the sur- 

 face of the sun, obviously cannot be shown; nor can the melee of 

 motions about a sunspot be made clear in any way except by pro- 

 jected motion pictures. These sample photographs have been chosen, 

 in most cases, to emphasize changes in form rather than details of 

 structure, as well as to indicate the importance of obtaining a large 

 nimiber of observations in a short space of time in the investigations 

 of many prominence motions. 



A very familiar type of solar photograph is reproduced in plate 

 1, A, which gives the appearance of the surface of the sun on August 

 18, 1939, showing a number of sunspots of medium size. This is an 

 image of the intensely brilliant photosphere — the surface studied 

 by the earliest visual observers and, except at times of solar eclipses, 

 the only surface available until the advent of the spectroscope. The 

 advantage in the ability to use a selected wave length for observation 

 is seen in plate 1, B, a spectroheliogram of the chi'omosphere taken 

 in the light of the hydrogen line Ha a few moments after the direct 

 photograph A was exposed. In this case bright and dark markings 

 in the chromosphere are clearly delineated and serve to indicate 

 features of tliis higher layer of gases which are transparent to in- 

 tegrated light and, consequently, do not register on a direct photo- 

 graph. Many of these dark markings or "flocculi" drift across the 

 disk with the rotation of the sun, showing only slight changes over 

 a long period of time. But often, especially in disturbed areas as- 

 sociated with sunspots, a dark flocculus will suddenly burst into 



