But you must please here observe that while my first diagram did with some adequateness represent to you the color facts there spoken of, the present diagram can only _explain_, not reproduce them. The bright reflected colors of clouds _can_ be represented in painting, because they are relieved against darker colors, or, in many cases, _are_ dark colors, the vermilion and ruby clouds being often much darker than the green or blue sky beyond them. But in the case of the phenomena now under your attention, the colors are all _brighter than pure white_,--the entire body of the cloud in which they show themselves being white by transmitted light, so that I can only show you what the colors are, and where they are,--but leaving them dark on the white ground. Only artificial, and very high illumination would give the real effect of them,--painting cannot.
Enough, however, is here done to fix in your minds the distinction between those two species of cloud,--one, either stationary,[16] or slow in motion, _reflecting unresolved_ light; the other, fast-flying, and _transmitting resolved_ light. What difference is there in the nature of the atoms, between those two kinds of clouds? I leave the question with you for to-day, merely hinting to you my suspicion that the prismatic cloud is of finely-comminuted water, or ice,[17] instead of aqueous vapor; but the only clue I have to this idea is in the purity of the rainbow formed in frost mist, lying close to water surfaces. Such mist, however, only becomes prismatic as common rain does, when the sun is behind the spectator, while prismatic clouds are, on the contrary, always between the spectator and the sun.
The main reason, however, why I can tell you nothing yet about these colors of diffraction or interference, is that, whenever I try to find anything firm for you to depend on, I am stopped by the quite frightful inaccuracy of the scientific people's terms, which is the consequence of their always trying to write mixed Latin and English, so losing the grace of the one and the sense of the other. And, in this point of the diffraction of light I am stopped dead by their confusion of idea also, in using the words undulation and vibration as synonyms. "When," says Professor Tyndall, "you are told that the atoms of the sun _vibrate_ at different rates, and produce _waves_ of different sizes,--your experience of water-waves will enable you to form a tolerably clear notion of what is meant."