From the consideration of the proprieties of which impressions, we may collect these short definitions of Colours:  That Blue is an impression on the Retina of an oblique and confus’d pulse of light, whose weakest part precedes, and whose strongest follows. And, that Red is an impression on the Retina of an oblique and confus’d pulse of light, whose strongest part precedes, and whose weakest follows.

Which proprieties, as they have been already manifested, in the Prisme and falling drops of Rain, to be the causes of the colours there generated, may be easily found to be the efficients also of the colours appearing in thin laminated transparent bodies; for the explication of which, all this has been premised.

And that this is so, a little closer examination of the Phaenomena and the Figure of the body, by this Hypothesis will make evident.

For first (as we have already observed) the laminated body must be of a determinate thickness, that is, it must not be thinner then such a determinate quantity; for I have always observ’d, that neer the edges of those which are exceeding thin, the colours disappear, and the part grows white; nor must it be thicker then another determinate quantity; for I have likewise observ’d, that beyond such a thickness, no colours appear’d, but the Plate looked white, between which two determinate thicknesses were all the colour’d Rings; of which in some substances I have found ten or twelve, in others not half so many, which I suppose depends much upon the transparency of the laminated body.  Thus though the consecutions are the same in the scumm or the skin on the top of metals; yet in those consecutions in the same colour is not so often repeated as in the consecutions in thin Glass, or in Sope-water, or any other more transparent and glutinous liquor; for in these I have observ’d, Red, Yellow, Green, Blue, Purple; Red, Yellow, Green, Blue, Purple; Red, Yellow, Green, Blue, Purple; Red, Yellow, &c. to succeed each other, ten or twelve times, but in the other more opacous bodies the consecutions will not be half so many.

And therefore secondly, the laminated body must be transparent, and this I argue from this, that I have not been able to produce any colour at all with an opacous body, though never so thin.  And this I have often try’d, by pressing small Globule of Mercury between two smooth Plates of Glass, whereby I have reduc’d that body to a much greater thinness then was requisite to exhibit the colours with a transparent body.

Thirdly, there must be a considerable reflecting body adjacent to the under or further side of the lamina or plate:  for this I always found, that the greater that reflection was, the more vivid were the appearing colours.