First, know that Newton was NOT the first scientist to take an interest in the colors of soap films.
ROBERT HOOKE was. (R. Boyle fanboys might disagree but my opinion is that Boyle co-opted Hooke's work.)
From Hooke's MICROGRAPHIA, 1664 ::
"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 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 Phænomena 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."
Next: Newton Proves Hooke's Theory of Color is Wrong.
From THIS EXCELLENT PAGE: "In the late 1660s, Newton starts experimenting with his ’celebrated phenomenon of colors.’ At the time, people thought that color was a mixture of light and darkness, and that prisms colored light. Hooke was a proponent of this theory of color, and had a scale that went from brilliant red, which was pure white light with the least amount of darkness added, to dull blue, the last step before black, which was the complete extinction of light by darkness. Newton realizes this theory was false.
Newton set up a prism near his window, and projected a beautiful spectrum 22 feet onto the far wall. Further, to prove that the prism was not coloring the light, he refracted the light back together.
Artists were fascinated by Newton’s clear demonstration that light alone was responsible for color."
Interesting reading about the Hooke/Newton rivalry HERE.
Newton & Soap Films
In December 1675 Newton presented his treatise:
An Hypothesis explaining the Properties of Light to the Royal Society.
While most of the experimental results of his hypothesis depend upon apparatus such as thin glass plates (sandwiching thin layers of gases, water etc), oil slicks or heated metal. . . this is (my opinion) the best of Newton's writing about the colors of soap films.
This was later included in Newton's OPTICKS, published in 1704.
An Hypothesis explaining the Properties of Light. 1675
... "The precedent observations were made with a rarer thin medium terminated by a denser, such as was air or water compressed betwixt two glasses. In those, that follow, are set down the appearances of a denser medium thinned within a rarer; such as are the plates of Muscovy-glass, bubbles of water, and some others thin substances terminated on all sides with air.
Obs. 17. If a bubble be blown with water, first made tenacious by dissolving a little soap in it, it is a common observation, that after a while it will appear tinged with a great variety of colours. To defend these bubbles from being agitated by the external air (whereby their colours are irregularly moved one among another, so that no accurate observation can be made of them) as soon as I had blown any of them, I covered it with a clear glass, and by that means its colours emerged in a very regular order, like so many concentric rings incompassing the top of the bubble. And as the bubble grew thinner by the continual subsiding of the water, these rings dilated slowly, and overspread the whole bubble, descending in order to the bottom of it, where they vanished successively. In the mean while, after all the colours were emerged at the top, there grew in the center of the rings a small, round, black spot, like that in the first observation, which continually dilated itself, till it became sometimes more than one half or three fourths of an inch in breadth, before the bubble broke. At first I thought there had been no light reflected from the water in that place; but observing it more curiously, I saw within it several smaller, round spots, which appeared much blacker and darker than the rest, whereby I knew, that there was some reflection at the other places, which were not so dark as those spots. And by further trial I found, that I could see the images (as of a candle or the sun) very faintly reflected, not only from the great black spot, but also from the little darker spots, which were within it.
Besides the aforesaid coloured rings, there would often appear small spots of colours ascending and descending up and down the side of the bubble, by reason of some inequalities in the subsiding of the water; and sometimes small black spots generated at the sides, would ascend up to the larger black spot at the top of the bubble, and unite with it.
Obs. 18. Because the colours of these bubbles were more extended and lively than those of air thinned between two glasses, and so more easy to be distinguished, I shall here give you a further description of their order, as they were observed in viewing them by reflection of the skies, when of a white colour, whilst a black substance was placed behind the bubble: and they were these; red, blue, red, blue; red, blue; red, green; red, yellow; green, blue, purple; red, yellow, green, blue, violet; red, yellow, white, blue, black.
The three first successions of red and blue were very dilute and dirty, especially the first, where the red seemed in a manner to be white. Amongst these there was scarcely any other colour sensible, only the blues (and principally the second blue) inclined a little to green.
The fourth red was also dilute and dirty, but not so much as the former three: after that succeeded little or no yellow, but a copious green, which at first was inclined a little to yellow, and then became a pretty brisk and good willow green, and afterwards changed to a blueish colour; but there succeded neither blue nor violet.
The fifth red at first was very much inclined to purple, and afterwards became more bright and brisk, but yet not very pure. This was succeeded with a very bright and intense yellow, which was but little in quantity, and soon changed to green; but that green was copious, and something more pure, deep, and lively, than the former green. After that followed an excellent blue of a bright sky colour; and then a purple, which was less in quantity than the blue, and much inclined to red.
The sixth red was at first of a very fair and lively scarlet, and soon after of a brighter colour, being very pure and brisk, and the best of all the reds. Then, after a lively orange, followed an intense, bright, and copious yellow, which was also the best of all the yellows; and this changed, first to a greenish yellow, and then to a greenish blue; but the green between the yellow and blue was very little and dilute, seeming rather a greenish white than a green. The blue, which succeeded, became very good, and of a fair, bright, sky colour; but yet something inferior to the former blue: and the violet was intense and deep, with little or no redness in it, and less in quantity than the blue.
In the last red appeared a tincture of scarlet next the violet, which soon changed to a brighter colour, inclining to an orange: and the yellow, which followed, was at first pretty good and lively, but afterwards it grew more and more dilute, until by degrees it ended in perfect whiteness: and this whiteness, if the water was very tenacious and well tempered, would slowly spread and dilate itself over the greatest part of the bubble, continually growing paler at the top, where at length it would crack, and those cracks, as they dilated, would appear of a pretty good, but yet obscure and dark, sky-colour; the white between the blue spots diminishing, until it resembled the threads of an irregular net-work, and soon after vanished and left all the upper part of the bubble of the said dark blue colour; and this colour, after the aforesaid manner, dilated itself downwards, until sometimes it hath overspread the whole bubble. In the mean while, at the top, which was of a darker blue than the bottom, and appeared also of many round blue spots, something darker than the rest, there would emerge one or more very black spots, and within those, other spots of an intenser blackness, which I mentioned in the former observation; and those continually dilated themselves until the bubble broke.
If the water was not very tenacious, the black spots would break forth in the white, without any sensible intervention of the blue: and sometimes they would break forth within the precedent yellow, or red, or perhaps within the blue of the second order, before the intermediate colours had time to display themselves.
By this description you may perceive, how great an affinity these colours have with those of air, described in the fourth observation, although set down in a contrary order, by reason that they begin to appear, when the bubble is thickest, and are most conveniently reckoned from the lowest and thickest part of the bubble upwards.
Obs. 19. Viewing, at several oblique positions of my eye, the rings of colours emerging on the top of the bubble, I found, that they were sensibly dilated by increasing the obliquity, but yet not so much by far, as those made by thinned air in the seventh observation. For there they distended so much, as, when viewed most obliquely, to arrive at a part of the plate more than twelve lines thicker than that where they appeared, when viewed perpendicularly; whereas in this case the thickness of the water, at which they arrived when viewed most obliquely, was, to that thickness, which exhibited them by perpendicular rays, something less than eight to five. By the best of my observations, it was between fifteen and fifteen and a half to ten, an increase about twenty-four times less than in the other case.
Sometimes the bubble would become of an uniform thickness all over, except at the top of it near the black spot, as I knew, because it would exhibit the same appearance of colours in all positions of the eye; and then the colours, which were seen at its apparent circumference by the obliquest rays, would be different from those, that were seen in other places by rays less oblique to it. And divers spectators might see the same part of it by differing colours, by viewing it at very differing obliquities. Now, observing how much the colours at the same place of the bubble, or at divers places of equal thickness, were varied by the several obliquities of the rays, by assistance of the fourth, fourteenth, sixteenth, and eighteenth observations, as they are hereafter explained, I collected the thickness of the water, requisite to exhibit any one the same colour at several obliquities, to be very nearly in the proportion expressed in this table.
In the two first columns are expressed the obliquities of the rays to the superficies of the water; that is, their angles of incidence and refraction; where, I suppose, that the lines, which measure them, are in round numbers, as three to four, though probably the dissolution of soap in the water may a little alter its refractive virtue. In the third column the thickness of the bubble, <284> at which any one colour is exhibited in those several obliquities, is exprest in parts, of which ten constitute that thickness, when the rays are perpendicular."
Source: Thomas Birch, The History of the Royal Society, vol. 3 (London: 1757), pp. 247-305. This Page.