The advance of photography : its history and modern applications (1911)

ASTRONOMICAL PHOTOGRAPHY 291 the sun at Iowa, in North America, and more than thirty telescopes were set up to retain the phenomenon. By these observations, the question respecting the nature of the protuberances was finally set at rest, and the only question that remained related to the corona. By corona is meant a kind of nimbus of white light surrounding the sun when totally eclipsed. Many observations of total eclipses have been undertaken to decide its nature. A very beautiful view of the corona was obtained by Whipple, at Shelby ville in Kentucky, August 7th 1869. The feeble light of this phenomenon renders a much longer exposure necessary than in the case of the protuberances. At Shelby ville, the exposure for the corona lasted fortytwo seconds, whereas five seconds sufficed to take the protuberances. Nor was the nature of the corona as yet determined. In 1870 an English expedition, conducted by Lockyer, was sent to Catania to observe the corona, and Dr Vogel accompanied it. Unfortunately, owing to the unfavourable weather, the observations were only partially successful. However, a detachment of the expedition, conducted by Brother, in Syracuse, succeeded in obtaining a good view of the corona, and the illustration (Fig. 120) is copied from this photograph. The black prominences round the sun's disc give the situation of the protuberances which were visible on the day of the eclipse. We call attention to the fact that they are not visible in the photograph of the corona. To take a view of the corona requires an exposure eight times as long as for the protuberances. During this long exposure the images of the protuberances received too much light, and have therefore become paler instead of brighter, so that their outline becomes confounded with the less bright parts. To show to what an extent these solar-eclipse expeditions have grown, and also to give the reader an opportunity