Page:Popular Science Monthly Volume 57.djvu/24

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14
POPULAR SCIENCE MONTHLY.

2.0 seconds only, generally one second being about right; the wet plate will take the corona in eight seconds or less. The best time of exposure should be tested on a bright star of about the second magnitude, by trial before the eclipse. There is no rule about the photographic focus, except to discover it by a series of exposures at different distances near the supposed point. Eclipse work is a practical matter, and many rough-and-ready methods must necessarily be admitted. A good lens in a wooden light-tight tube, supported at each end, having the motion of the sun, the photographic focal plane carefully determined, the time exposures very short, and, finally, exceedingly slow development of the picture after the eclipse—these form the prime requisites. Expensive telescopes, clockwork on heavy iron piers, reflecting mirrors, and such like apparatus are not needed. Ingenuity in practical details, with great anxiety about the essential matter of the light itself, is what is needed for a successful eclipse expedition.

Those persons who have no telescope for viewing the sun, or camera for photographing it, can yet see the corona to great advantage by means of a good opera glass, and indeed this is really the most satisfactory way to thoroughly enjoy the spectacle. The object may be sketched on paper at once or from memory, and this picture may well be of value to astronomy.

The tracks of the eight North American eclipses seen since 1800 are shown on Chart VI. It is noted that three have paths very similarly located, and that five run in directions about parallel to one another, but almost at right angles to the first group. This comprises the eclipse of November 30, 1834, duration two minutes; August 7, 1869, two minutes and three quarters; July 29, 1878, two minutes and a half, which stretch from Alaska south-eastward in a fan-shape to the South Atlantic coast. The second group contains the tracks June 16, 1806, four minutes and a half; July 17, 1860, three minutes; January 11, 1880 thirty-two seconds; January 1, 1889, two minutes and a quarter; and May 28, 1900, two minutes. These tracks all trend from southwest to northeast, and cross the North American continent in different latitudes, that of May 28, 1900, being the most southerly and of rather short duration, lasting less than two minutes in the United States.

The path of the total eclipse of May 28, 1900, after leaving the United States, crosses the North Atlantic Ocean to Coimbra, Portugal, and continues over North Africa to its end at the Red Sea. Stations which are not situated on the path of the totality will see the sun partially eclipsed, in proportion to the distance of the locality from the central line to the northern or the southern lim-