Page:Popular Science Monthly Volume 65.djvu/110

106 position on the sun's limb. It was exceedingly unfortunate that three cameras of the forty-foot pattern could not have been working in harmony, at three stations widely separated, to determine what changes, if any, were taking place in the disturbed coronal area. Under excellent atmospheric conditions, cameras still larger than those referred to should record more minute details of coronal structure, and thus lead to valuable results; but such observations would reach their full value only in case comparisons could be instituted with photographs taken under similar conditions at distant stations. However, as already stated, cameras of the forty-foot pattern give greater promise of cooperative usefulness, taking into account the average atmospheric conditions which must be expected at some of the stations.

The spectra of recent coronas have led to most interesting results. They leave no doubt that, at those eclipses, the spectrum of the inner corona contained no perceptible dark lines. Perrine's Sumatra photographs seem to establish that the spectrum of the great outer portion is substantially a copy of the solar spectrum. The simplest interpretation of these observations is that the outer corona is largely composed of minute particles which reflect and diffract the sunlight falling upon them, whereas the portions near the hot solar surface are mostly incandescent, shining by their own light. Polarigraphic observations are in harmony with this theory. Opposed to the idea of the incandescence of the inner corona stands, alone, the thermographic observations by Abbot in 1900, of a corona less hot than the instrument with which he worked. While it is difficult to assign such a low temperature to particles near the solar surface, and one should perhaps look for other interpretations of the thermographic results, yet there is an urgent demand for a repetition of all the preceding observations bearing upon the nature of the corona.

The polarigraphic observations of recent coronas have been very interesting—leading to the knowledge that the light of the corona is strongly polarized, except, apparently, in close proximity to the sun's surface; and strengthening the view that the corona is very largely composed of minute particles of matter which receive their light from the photosphere. Unfortunately, the photographs do not permit the making of quantitative measurements of the amount of polarization in and across the solar radii; and future programs for eclipse observations in this line should make provision for securing comparable unpolarized coronal images for standards of reference.

Special interest will be taken in determining whether the comparatively shallow inner stratum of the corona which yields a brightline spectrum, is more extensive at the sunspot maximum of 1905 than it was at the minimum of 1898–1901. The chances are that it will be both thicker and more uniform in thickness. Should it be brighter