To begin therefore, it is manifest from several circumstances, that the material cause of the apparition of these several Colours, is some Lamina or Plate of a transparent or pellucid body of a thickness very determinate and proportioned according to the greater or less refractive power of the pellucid body. And that this is so, abundance of Instances and particular Circumstances will make manifest.
As first, if you take any small piece of the Muscovy-glass, and with a Needle, or some other convenient Instrument, cleave it oftentimes into thinner and thinner Laminæ, you shall find, that till you come to a determinate thinness of them, they shall all appear transparent and colourless, but if you continue to split and divide them further, you shall find at last, that each Plate, after it comes to such a determinate thickness, shall appear most lovely ting'd or imbued with a determinate colour. If further, by any means you so flaw a pretty thick piece, that one part does begin to cleave a little from the other, and between those two there be by any means gotten some pellucid medium, those laminated pellucid bodies that fill that space, shall exhibit several Rainbows or coloured Lines, the colours of which will be disposed and ranged according to the various thicknesses of the several parts of that Plate. That this is so, is yet further confirmed by this Experiment.
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 Laminæ 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 Phænomena of Colours: which Phænomena 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 Phænomena, I had often
