CORONA PHOTOGRAPHY The corona is the brilliant pearly-white lumin escence observed round the eclipsed sun during a total solar eclipse, when the ordinary brilliant sunlight is obscured by the intervention of the dark moon. Many attempts have been made by astronomers to observe it visually or to photo graph it in ordinary sunlight, but all these have hitherto been unsuccessful, and it is only during an eclipse that it can be examined. There is no limit, either of size or quality, to the apparatus.
which can be usefully employed to photograph this wonderful solar appendage, and as indicat ing what are perhaps extremes, excellent pic tures of the coronal streamers have been made with ordinary camera lenses of about 5 in. focal length, and also with large mirror telescopes of more than 7o ft. focal length. Each variety, however, has its special advantages, and the general apparatus may be divided into two divi sions according to the scale on which the pic tures are primarily photographed. With the small scale cameras, lenses of very great angular aperture can be employed, with all the advan tages of extensive flat field and light intensity. These will be most useful for recording the extreme limits of the coronal streamers or rays, which are found to be very different from eclipse to eclipse, so that as complete a record as pos sible is desirable.
With the large scale apparatus the programme is best confined to obtaining detailed pictures of the brighter regions of the corona near to the sun's disk, and as the light gradations in this region are very great, it is generally found neces sary to take exposures of different lengths for the different zones, and develop accordingly, so that the intense portions are not rendered unprintable.
If possible, the most rapid panchromatic plates should be employed for photographing the corona, as the principal part of the corona radiations is in the green near wave length 5,303.
The exposures possible will, of course, depend on whether the observer has the use of an equa torial mounting and driving mechanism or not. If he has not, probably half a second will be the maximum exposure possible with a camera of about 7 in. to 9 in. focal length. When the
more elaborate mounting is available, exposures of from ten seconds to four or five minutes are given so as to record the farthest outlying streamers.
Very interesting series of experiments have been tried at several eclipses. For comparison purposes two exactly similar photographic cameras have been used side by side, damped firmly together so as to move as one. In front of one of these cameras, between the lens and the corona, there has been placed a special piece of apparatus for detecting polarised light, so that if any of the light coming from the solar corona is polarised in any particular direction, the differ ence will be clearly shown on the photograph by the presence of a series of bands as compared with the image taken with the other camera having the lens alone. By such means it has been satisfactorily proved that a large proportion of the light from the corona is polarised, and it is thought that this may be owing to its being light from the sun reflected from the minute particles of which the corona is assumed to be composed.
By obtaining comparable photographs with the same instruments at intervals during a long period of years it has been noticed that the form of the corona decidedly changes ; and these changes are found to be in accord with the changes on the sun's surface as evidenced by the presence or absence of the dark irregular markings called sunspots. When these are very numerous, showing the sun to be very active, then the corona is found to be very extensive in all directions round the sun, long streamers passing off practically in all directions. When, on the contrary, the sun's activity is at a minimum, and there are few sunspots, then the coronal streamers are chiefly confined to the regions on either side of the sun's equator ; the regions near the sun's poles are at these times occupied by very beau tiful plumes or aigrettes, which have all the appearance of the stream lines shown by a series of filings congregating about the poles of a magnet. This fact has given considerable prob ability to the suggestion that the solar corona is due to some electromagnetic discharge from the sun's surface.