position which flat plates finally assume when falling through quiescent air.  We shall presently consider what the conditions must be, in order that the crystals may be liable to be now and then disturbed from the horizontal position.  If this occasionally happens, the crystals will keep fluttering, and at any one moment some of them will be turned so as to reflect a ray from the sun to the eye of the observer from the flat surface of the crystal which is next him.  Now, if the conditions are such as to produce crystals which are plates with parallel faces, and as they are also transparent, part only of the sun’s ray that reaches the front face of the crystal will be reflected from it; the rest will enter the crystal, and, falling on the parallel surface behind, a portion will be there reflected, and passing out through the front face, will also reach the eye of the observer.

These two portions of the ray—­that reflected from the front face and that reflected from the back—­are precisely in the condition in which they can interfere with one another, so as to produce the splendid colors with which we are familiar in soap bubbles.  If the crystals are of diverse thicknesses, the colors from the individual crystals will be different, and the mixture of them all will produce merely white light; but if all are nearly of the same thickness, they will transmit the same color toward the observer, who will accordingly see this color in the part of the cloud occupied by these crystals.  The color will, of course, not be undiluted; for other crystals will send forward white light, and this, blended with the colored light, will produce delicate shades in cases where the corresponding colors of a soap bubble would be vivid.

We have now only to explain how it happens that on very rare occasions the colors, instead of lying in irregular patches, form definite fringes round the borders of the cloudlets.  The circumstances that give rise to this special form of the phenomenon appear to be the following:  While the cloud is in the process of growth (that is, so long as the precipitation of vapor into the crystalline state continues to take place), so long will the crystals keep augmenting.  If, then, a cloudlet is in the process of formation, not only by the springing up of fresh crystals around, but also by the continued growth of the crystals within it, then will that patch of cloud consist of crystals which are largest in its central part, and gradually smaller as their situation approaches the outside.  Here, then, are conditions which will produce one color round the margin of the cloud, and that color mixed with others, and so giving rise to other tints, farther in.  In this way there comes into existence that iris-like border which is now and then seen.