Page:Popular Science Monthly Volume 60.djvu/257

Rh way on the summit of El Misti, Peru, at an elevation of 19,200 feet. At this altitude one-half the earth's atmosphere is below the observer and that which remains is of extraordinary clearness. Photographs were made of the region immediately about the sun, using an opaque disc to protect the plate from the sun's direct image. The true corona did not appear upon the plates. Other methods promised better results, such as the use of monochromatic light, presumably that of the line 'K 1474.' Experiments in this line have been carried on by Professor Hale with skill and enthusiasm on the summit of Pike's Peak, on Mount Etna and elsewhere, but without success. He has also attempted to solve the difficulty by a study of the heat, using the bolometer. Recent investigations given below explain the failure

of this method. The polarization of the coronal light also suggests a method which has not yet yielded successful results. Although the future may furnish the solution, none of the attempts yet made has been successful, and for the present our only knowledge of the corona must be obtained from what can be learned during the brief moments of total eclipses. Good photographs of the corona can be easily and rapidly made and if an abundance of these were alone necessary our knowledge would be well advanced. The general features of the corona have a certain permanence. Comparatively slight changes are known to take place during the three or four hours while an eclipse is passing over the surface of the earth. There may be, however, finer details than are shown on the best photographs yet obtained, which