For spectroscopic observations at the edge of the sun, on the contrary, the _sine quâ non_ is translucency. During the great “Indian eclipse” of August 18th, 1868, the variously colored lines were, by the aid of prismatic analysis, first described, which reveal the chemical constitution of the flamelike “prominences,” forming an ever-varying, but rarely absent, feature of the solar surroundings. Immediately afterwards, M. Janssen, at Guntoor, and Mr. Norman Lockyer, in England, independently realised a method of bringing them into view without the co-operation of the eclipsing moon. This was done by _fanning out_ with a powerfully dispersive spectroscope the diffused radiance near the sun, until it became sufficiently attenuated to permit the delicate flame-lines to appear upon its rainbow-tinted background. This mischievous radiance—which it is the chief merit of a solar eclipse to abolish during some brief moments—is due to the action of the atmosphere, and chiefly of the watery vapors contained in it. Were our earth stripped of its “cloud of all-sustaining air,” and presented, like its satellite, bare to space, the sky would appear perfectly black up to the very rim of the sun’s disc—a state of things of all others (vital necessities apart) the most desirable to spectroscopists. The best approach to its attainment is made by mounting a few thousand feet above the earth’s surface. In the drier and purer air of the mountains, “glare” notably diminishes, and the tell-tale prominence-lines are thus more easily disengaged from the effacing lustre in which they hang, as it were suspended. The Peak of Teneriffe, as we have seen, offers a marked exception to this rule, the impalpable dust diffused through the air giving, even at its summit, precisely the same kind of detailed reflection as aqueous vapors at lower levels. It is accordingly destitute of one of the chief qualifications for serving as a point of vantage to observers of the new type. The changes in the spectra of chromosphere and prominences (for they are parts of a single appendage) present a subject of unsurpassed interest to the student of solar physics. There, if anywhere, will be found the key to the secret to the sun’s internal economy; in them, if at all, the real condition of matter in the unimaginable abysses of heat covered up by the relatively cool photosphere, whose radiations could, nevertheless, vivify 2,300,000,000 globes like ours, will reveal itself; revealing, at the same time, something more than we know of the nature of the so-called “elementary” substances, hitherto tortured, with little result, in terrestrial laboratories.