Take two small pieces of ground and polisht Looking-glass-plate, each about the bigness of a shilling, take these two dry, and with your fore-fingers and thumbs press them very hard and close together, and you shall find, that when they approach each other very near, there will appear several Irises or coloured Lines, in the same manner almost as in the Muscovy-glass; and you may very easily change any of the Colours of any part of the interposed body, by pressing the Plates closer and harder together, or leaving them more lax; that is, a part which appeared coloured with a red, may be presently ting’d with a yellow, blew, green, purple, or the like, by altering the appropinquation of the terminating Plates.  Now that air is not necessary to be the interposed body, but that any other transparent fluid will do much the same, may be tryed by wetting those approximated Surfaces with Water, or any other transparent Liquor, and proceeding with it in the same manner as you did with the Air; and you will find much the like effect, only with this difference, that those comprest bodies, which differ most, in their refractive quality, from the compressing bodies, exhibit the most strong and vivid tinctures.  Nor is it necessary, that this laminated and ting’d body should be of a fluid substance, any other substance, provided it be thin enough and transparent, doing the same thing:  this the Laminae of our Muscovy-glass hint; but it may be confirm’d by multitudes of other Instances.

And first, we shall find, that even Glass it self may, by the help of a Lamp, be blown thin enough to produce these Phaenomena of Colours:  which Phaenomena accidentally happening, as I have been attempting to frame small Glasses with a Lamp, did not a little surprize me at first, having never heard or seen any thing of it before; though afterwards comparing it with the Phaenomena, I had often observed in those Bubbles which Children use to make with Soap-water, I did the less wonder; especially when upon Experiment I found, I was able to produce the same Phaenomena in thin Bubbles made with any other transparent Substance.  Thus have I produced them with Bubbles of Pitch, Rosin, Colophony, Turpentine, Solutions of several Gums, as Gum-Arabick in water; any glutinous Liquor, as Wort, Wine, Spirit of Wine, Oyl of Turpentine, Glare of Snails, &c.

It would be needless to enumerate the several Instances, these being enough to shew the generality or universality of this propriety.  Only I must not omit, that we have instances also of this kind even in metalline Bodies and animal; for those several Colours which are observed to follow each other upon the polisht surface of hardned Steel, when it is by a sufficient degree of heat gradually tempered or softened, are produced, from nothing else but a certain thin Lamina of a vitrum or vitrified part of the Metal, which by that degree of heat, and the concurring action of the ambient Air, is driven out and fixed on the surface of the Steel.