Page:Popular Science Monthly Volume 65.djvu/111

Rh than at recent eclipses, the opportunity to search for new coronal bright lines will be excellent.

The accurate wave-length of the principal coronal bright line, near λ 5303, should be determined. A modern spectrograph, holding three dense flint prisms, should make the problem easy. The accurate wavelengths of all truly coronal lines should be determined as rapidly as possible, partly in order that a serious effort may be made to represent them by a simple common law, as has been done for hydrogen and helium.

Of many other eclipse problems—the photometry, the shadow bands, etc.—it need only be said that accurate observations will prove very useful.

The tendency of recent eclipse work is toward a unification of the problem. The main divisions of the sun's structure are no longer to be studied separately. Close connection has been observed between spots and faculæ; between photosphere and reversing layer; between sun spot and coronal disturbances; between coronal streamers and prominences; between prominences and chromosphere; between the sunspot curve and the form of the corona; and in other ways the unity of the problem is emphasized. This is only what we should expect, for all these outward and visible features of the sun must be related products of the stupendous forces at work within its body. In reality, all observations of the sun, whether those made daily at fixed observatories or those secured at eclipses, bear upon the solution of one problem: the structure, composition and condition of the sun, from its center to the outermost limits of the coronal streamers.

It is well known to eclipse observers that a regrettably large proportion of observations of these phenomena are failures, or are but partially successful. Some of these unfortunate outcomes are due to nervousness at the critical moment; a psychological state of which some observers know nothing, and against which others are unable to contend. It is a mistake to invite nervousness by attempting to do too much in the limited duration of totality. If seven photographs can be secured with one instrument, working with moderate speed in changing plates, an attempt to secure eight by working under high nervous tension would be a serious error. However, the most prolific source of failure is that of new instruments and new methods used for the first time on eclipse day. It is not an uncommon practise to delay preparations until a few months or weeks before expeditions must depart for their stations; to order new instruments, or new parts of instruments, just in time to have them shipped from factory to station; to use new methods of focusing, etc., for the first time, at the station; and to leave insufficient time for the rehearsal of program after the instruments are in supposed adjustment. It is unnecessary to say that this is