They appear from various causes on the surface of a colourless body, originally, without communication, die or immersion (βαφή); and we now proceed to trace them, from their faintest indication to their most permanent state, through the different conditions of their appearance, which for easier survey we here at once summarily state. 431. First condition.--The contact of two smooth surfaces of hard transparent bodies. First case: if masses or plates of glass, or if lenses are pressed against each other. Second case: if a crack takes place in a solid mass of glass, chrystal, or ice. Third case: if lamellæ of transparent stones become separated. Second condition.--If a surface of glass or a polished stone is breathed upon. Third condition.--The combination of the two last; first, breathing on the glass, then placing another plate of glass upon it, thus exciting the colours by pressure; then removing the upper glass, upon which the colours begin to fade and vanish with the breath. Fourth condition.--Bubbles of various liquids, soap, chocolate, beer, wine, fine glass bubbles. Fifth condition.--Very fine pellicles and lamellæ, produced by the decomposition of minerals and metals. The pellicles of lime, the surface of stagnant water, especially if impregnated with iron, and again pellicles of oil on water, especially of varnish on aqua fortis. Sixth condition.--If metals are heated; the operation of imparting tints to steel and other metals. Seventh condition.--If the surface of glass is beginning to decompose. 432. First condition, first case. If two convex glasses, or a convex and plane glass, or, best of all, a convex and concave glass come in contact, concentric coloured circles appear. The phenomenon exhibits itself immediately on the slightest pressure, and may then be gradually carried through various successive states. We will describe the complete appearance at once, as we shall then be better enabled to follow the different states through which it passes. 433. The centre is colourless; where the glasses are, so to speak, united in one by the strongest pressure, a dark grey point appears with a silver white space round it: then follow, in decreasing distances, various insulated rings, all consisting of three colours, which are in immediate contact with each other. Each of these rings, of which perhaps three or four might be counted, is yellow on the inner side, blue on the outer, and red in the centre. Between two rings there appears a silver white interval. The rings which are farthest from the centre are always nearer together: they are composed of red and green without a perceptible white space between them. 434. We will now observe the appearances in their gradual formation, beginning from the slightest pressure. 435.