that the calcium vapor usually coincides closely in form and position with the faculæ, and hence the calcium clouds were long spoken of under this name. In the new work at the Yerkes Observatory the distinction between the calcium clouds and the underlying faculæ is so marked that a distinctive name for the vaporous clouds has become necessary. They have therefore been designated flocculi, a name chosen without reference to their actual nature, but suggested by the flocculent appearance of the photographs.
In order to analyze these flocculi and to determine their true structure, a method was desired which would permit sections of them at different heights above the photosphere to be cut off, as it were, and
photographed. Fortunately there is a simple means of accomplishing this apparently difficult object. At the base of the flocculi the calcium vapor, just rising from the sun's interior, is comparatively dense. As it passes upward through the flocculi it reaches a region of much lower pressure, and during the ascent it might be expected to expand and therefore to become less dense. Now we know from experiments in the laboratory that dense calcium vapor produces very broad spectral bands, and that as the density of the vapor is decreased these bands narrow down into fine sharp lines (Fig. 8). An examination of the solar spectrum will show that the H and A lines of calcium give evidence of the occurrence of this substance under widely different densities in the sun. The broad dark bands, which for convenience we designate as H1 and K1, are due to the low-lying dense calcium vapor (Fig. 1). At their middle points (over flocculi) are seen two bright