148 Scientific Intelligence. 



which Hat 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 dis- 

 turbed 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. iS'ow, if the conditions are such as to produce crystals 

 which are plates with parallel faces, and as they are also trans- 

 parent, 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 crys- 

 tal, 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 thick- 

 nesses 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 correspond- 

 ing 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 circum- 

 stances that give rise to this special form of the phenomenon ap- 

 pear 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 forma 

 tion, 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. 



The occasional upsetting of the crystals, which is required to 

 keep them fluttering, may be produced in any of three ways. 

 The cloudlets may have been formed from the blending together 

 of two layers of air saturated at different temperatures, and mov- 

 ing with different velocities or in different directions. Where 

 these currents intermix a certain amount of disturbance will pre- 



