Philosophy 167: Class 11 - Part 11 - Correcting Chromatic Aberration: Newton and the Refraction Indices of Different Colors.

Smith, George E. (George Edwin), 1938-


  • Synopsis: In 1672, Newton published an explanation of chromatic aberration, and a new telescope design (the first reflecting telescope) which does not have chromatic aberration.

    Opening line: "It's in 1672, while Roche is down in Cayenne, that Newton publishes his first paper."

    Duration: 4:38 minutes.

    Segment: Class 11, Part 11.
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It's in 1672, while Roche is down in Cayenne, that Newton publishes his first paper. This is not his first paper, it's a second or third paper, but we'll look at the earlier papers next week. They're all in the collection, all appearing in 1672, 73. This is February. For those who don't know, technically the year started in March in those days.
But the English would normally list it as 72, 73 until you get to April. So this is technically 73 even though it's listed as 72 because there's ambiguity on when the new year is. Newton was already reasonably well known. His sequence, just to tell you, is he graduated in 1667.
And Barrow decided he didn't want to be a mathematician anymore. He wanted to be a minister, a theologian, and he recommended young Isaac Newton then, and he was about 27 years old, to assume the Lucasian professorship. By that time, Barrow knew, and a few other people knew that Newton was the leading mathematician in the world but he had not published anything at the time, but the reputation was there.
Newton was a major mathematician. In early 1672, he sends the first of a series of papers, as I say, this is the second. The first paper we'll talk about next week. It announces the phenomenon of chromatic aberration. Technically what it announces is the following, light, white light, is made up of light of many colors, and each of those colors has a different index of refraction.
So as a result, when you put it through a lens, you do not keep, there's an oblong broadening of the image. And corresponding to that is a splitting of the colors. That's the first paper. The second paper then presents the first reflecting telescope, the whole point of which was it eliminates chromatic aberration by not using refraction.
And, it was Barrow himself who carried this telescope down. It sits now in a cabinet at the Royal Society, so you can at least stare at it. It looks not very different from this drawing of it as a matter of fact, which is the drawing at the Royal Society.
The eyepiece is up here. There's a mirror down here, concave mirror. He brings the light, collects the light from a fairly large aperture. As you can see, it's the size of the one, the telescope in my office is a Newtonian reflector, brings it back up here and you look through an eyepiece at the image.
And the images, what they remark at here, much, much sharper than they had ever seen in a telescope before. Anyway, this is Newton bursting on the scene. We'll talk about it next week. The reason it's significant is for the first time there's actually a full understanding of what chromatic aberration is.
Newton says in this, chromatic aberration cannot be corrected in a refracting telescope, but Huygens already knew better. He didn't know what was causing chromatic aberration until Newton published. But after that, everybody knew. And Huygens hadn't realized that he had already done something toward correcting it, by in effect redirecting the rays.
One of Newton's experiments is to split the rays of different colors. So you've got each one as a separate color. Then bring them back, and form white light again. And so if you can do that, that's similar to what the Huygens eyepiece does. Okay, is it splits the rays and brings them back.
We'll talk about Newton next week, including all of his work in optics at the time, most of which, there are these initial publications, then he drops out. He gets into a big controversy over his work in optics. He doesn't like controversy, so he simply quits publishing entirely for roughly a ten year period.
The Principia is the next thing he publishes in life, after these papers in optics.