My rating: 4 of 5 stars
My Design in this Book is not to explain the Properties of Light by Hypotheses, but to propose and prove them by Reason and Experiment
I’ve long wanted to read Newton’s Principia, but its reputation intimidates me. Everyone seems to agree that it is intensely difficult, and I’m sorry to say I haven’t worked up enough nerve to face it yet. But I did still want to read Newton; so as soon as I learned about this book, Newton’s more popular and accessible volume, I snatched it up and happily dug in.
The majority of this text is given over to descriptions of experiments. To the modern reader—and I suspect to the historical reader as well—these sections are remarkably dry. In simple yet exact language, Newton painstakingly describes the setup and results of experiment after experiment, most of them conducted in his darkened chamber, with the window covered up except for a small opening to let in the sunlight. Yet even if this doesn’t make for a thrilling read, it is impossible not to be astounded at the depth of care, the keenness of observation, and the subtle brilliance Newton displays. Using the most basic equipment (his most advanced tool is the prism), Newton almost literally tweezes light apart, making an enormous contribution both to experimental science and to the field of optics.
At the time, the discovery that white light could be decomposed into a rainbow of colors, and that this rainbow could be recombined back into white light, must have seemed as momentous as the discovery of the Higgs Boson. And indeed, even the modern reader might catch a glimpse of this excitement as she watches Newton carefully set up his prism in front of his beam of light, tweaking every variable, adjusting every parameter, measuring everything could be measured, and describing in elegant prose everything that couldn’t.
Whence it follows, that the colorifick Dispositions of Rays are also connate with them, and immutable; and by consequence, that all the Productions and Appearances of Colours in the World are derived, not from any physical Change caused in Light by Refraction or Reflexion, but only from the various Mixtures or Separations of Rays, by virtue of their different Refrangibility or Reflexibility. And in this respect the Science of Colours becomes a Speculation as truly mathematical as any other part of Opticks.
Because I had recently read Feynman’s QED, one thing in particular caught my attention. Here’s the problem: When you have one surface of glass, even if most of the light passes through it, some of the light is reflected; and you can roughly gauge what portion of light does one or the other. Let’s say on a typical surface of glass, 4% of light is reflected. Now we add another surface of glass behind the first. According to common sense, 8% of the light should be reflected, right? Wrong. Now the amount of light which is reflected varies between 0% and 16%, depending on the distance between the two surfaces. This is truly bizarre; for it seems that the mere presence of second surface of glass alters the reflectiveness of the first. But how does the light “know” there is a second surface of glass? It seems the light somehow is affected before it comes into contact with either surface.
Well, Newton was aware of this awkward problem. He famously comes up with his theory of “fits of easy reflection or transmission” to explain this phenomenon. But this “theory” was merely to say that the glass, for some unknown reason, sometimes lets light through, and sometimes reflects it. In other words, it was hardly a theory at all.
Every Ray of Light in its passage through any refracting Surface is put into a certain transient Constitution or State, which in the progress of the Ray returns at equal Intervals, and disposes the Ray at every return to be easily transmitted through the next refracting Surface, and between the returns to be easily reflected by it.
Also fascinating to the modern reader is the strange dual conception of light as waves and as particles in this work, which can’t help but remind us of the quantum view. The wave theory makes it easy to account for the different refrangibility of the different colors of light (i.e. the different colors reflect at different angles in a prism).
Do not several sorts of Rays make Vibrations of several bignesses, which according to their bignesses excite Sensations of several Colours, much after the manner that the Vibrations of the Air, according to their several bignesses excite Sensations of several sounds. And particularly do not the most refrangible Rays excite the shortest Vibrations for making a Sensation of deep violet, the least refrangible the largest for making a Sensation of deep red, and the several intermediate bignesses to make Sensations of the several intermediate Colours?
To this notion of vibrations, Newton adds the “corpuscular” theory of light, which held (in opposition to his contemporary, Christiaan Huygens) that light was composed of small particles. This theory must have been attractive to Newton because it fit into his previous work in physics. It explained why beams of light, like other solid bodies, travel in straight lines (cf. Newton’s first law), and reflect off surfaces at angles equal to their angles of incidence (cf. Newton’s third law).
Are not the Rays of Light very small Bodies emitted from shining Substances? For such Bodies will pass through uniform Mediums in right Lines without bending into the shadow, which is the Nature of the Rays of Light. They will also be capable of several Properties, and be able to conserve their Properties unchanged in passing through several Mediums, which is another conditions of the Rays of Light.
As a side note, despite some problems with the corpuscular theory of light, it came to be accepted for a long while, until the phenomenon of interference gave seemingly decisive weight to the wave theory. (Light, like water waves, will interfere with itself, creating characteristic patterns; cf. the famous double-slit experiment.) The wave theory was reinforced with Maxwell’s equations, which treated light as just another electro-magnetic wave. It was, in fact, Einstein who brought back the viability of the corpuscular theory, when he suggested the idea that light might come in packets to explain the photoelectric effect. (Blue light, when shined on certain metals, will cause an electric current, while red light won’t. Why not?)
All this tinkering with light is good fun, especially if you’re a physicist (which I’m not). But the real treat, at least for the layreader, comes at the final section, where Newton speculates on many of the unsolved scientific problems of his day. His mind is roving and vast; and even if most of his speculations have turned out incorrect, it’s stunning to simply witness him at work. For example, Newton realizes that radiation can travel without a medium (like air), and can heat objects even in a vacuum. (And thank goodness for that, for how else would the earth be warmed by the sun?) But from this fact he incorrectly deduces that there must be some more subtle medium that remains (like the famous ether).
If in two large tall cylindrical Vessels of Glass inverted, to little Thermometers be suspended so as not to touch the Vessels, and the Air be drawn out of one of these Vessels thus prepared be carried out of a cold place into a warm one; the Thermometer in vacuo will grow warm as much, and almost as soon as the Thermometer that is not in vacuo. And when the Vessels are carried back into the cold place, the Thermometer in vacuo will grow cold almost as soon as the other Thermometer. Is not the Heat of the warm Room convey’d through the Vacuum by the Vibrations of a much subtiler Medium than Air, which after the Air was drawn out remained in the Vacuum?
Yet for all Newton’s perspicacity, the most touching section was a list of question Newton asks, as if to himself, that he cannot hope to answer. It seems that even the most brilliant among us are stunned into silence by the vast mystery of the cosmos:
What is there in places almost empty of Matter, and whence is it that the Sun and Planets gravitate towards one another, without dense Matter between them? Whence is it that Nature doth nothing in vain; and whence arises all that Order and Beauty which we see in the World? To what end are Comets, and whence is it that Planets move all one and the same way in Orbs concentrick, while Comets move all manner of ways in Orbs very excentrick; and what hinders the fix’d Stars from falling upon one another? How came the Bodies of animals to be contrived with so much Art, and for what ends were their several Parts? Was the Eye contrived without Skill in Opticks, and the Ear without Knowledge of Sounds? How do the Motions of the Body follow from the Will, and whence is the Instinct in Animals?