If, now, a prominence exists at this part of the solar limb, and if the spectroscope itself is so adjusted that the _C_ line falls in the centre of the field of view, then one will see something like Fig. 217. "The red portion of the spectrum will stretch athwart the field of view like a scarlet ribbon with a darkish band across it; and in that band will appear the prominences, like scarlet clouds, so like our own terrestrial clouds, indeed, in form and texture, that the resemblance is quite startling. One might almost think he was looking out through a partly-opened door upon a sunset sky, except that there is no variety or contrast of color; all the cloudlets are of the same pure scarlet hue. Along the edge of the opening is seen the chromosphere, more brilliant than the clouds which rise from it or float above it, and, for the most part, made up of minute tongues and filaments." 199. _Quiescent Prominences._--The prominences differ as widely in form and structure as in magnitude. The two principal classes are the _quiescent_, _cloud-formed_, or _hydrogenous_, and the _eruptive_, or _metallic_. [Illustration: Plate 3.] The _quiescent_ prominences resemble almost exactly our terrestrial clouds, and differ among themselves in the same manner. They are often of enormous dimensions, especially in horizontal extent, and are comparatively permanent, often undergoing little change for hours and days. Near the poles they sometimes remain during a whole solar revolution of twenty-seven days. Sometimes they appear to lie upon the limb of the sun, like a bank of clouds in the terrestrial horizon, probably because they are so far from the edge that only their upper portions are in sight. When fully seen, they are usually connected to the chromosphere by slender columns, generally smallest at the base, and often apparently made up of separate filaments closely intertwined, and expanding upward. Sometimes the whole under surface is fringed with pendent filaments. Sometimes they float entirely free from the chromosphere; and in most cases the larger clouds are attended by detached cloudlets. Various forms of quiescent prominences are shown in Plate III. Other forms are given in Figs. 218 and 219. [Illustration: Fig. 218.] Their spectrum is usually very simple, consisting of the four lines of hydrogen and the orange _D_^3: hence the appellation _hydrogenous_. Occasionally the sodium and magnesium lines also appear, even near the tops of the clouds. [Illustration: Fig. 219.]