Move forward through Book III. Much of it will be more or less incomprehensible, some of it brilliantly clear. The best is saved for last. The book, and the entire work, concludes with a General Scholium, into which Newton poured his soul.
The General Scholium, probably the most famous few pages in the history of science, asks very great questions. For the most part it does not answer them. Some of them are still unanswered today. They may never be answered. Isaac Newton died in 1727. Does he now know the answers, wherever he may be? I like to think so.
ISAAC NEWTON
Opticks
CHRISTIAAN HUYGENS
1619–1695
Treatise on Light
At the midpoint of the seventeenth century, two great scientific puzzles obsessed the best minds of Europe. One was very old, going back to Ptolemy and, indeed, to his ancient Greek forebears. How does the solar system work? What is the force that drives the planets, in what orbits, around what central point? Advances had been made on the problem by Copernicus in the fifteenth century and by Kepler, Gilbert, and Galileo in the late sixteenth and early seventeenth centuries. But the precise workings of the great system were not known, a fact that tormented the leading scientists of every modern country.
Sir Isaac Newton, the Englishman, solved this puzzle. How he did it is told, and the book he wrote about it described, in the preceding entry.
The other great seventeenth-century puzzle was new. It involved that most omnipresent of things—but was it a thing?—light. Light manifested very strange behaviors. Dozens of natural philosophers set up telescopes (thanks to Galileo) and used prisms and arranged black boxes. How did light work? What explained its well-known powers of reflection and refraction? What was color? What was light itself? Isaac Newton solved this puzzle, too.
Not by himself, however. He did not find all of the answer—even all of it that could be known in the late seventeenth century. The strange and puzzling behavior of light was beyond even his enormous capacity to understand simply. His solution of the puzzle of light was therefore incomplete and partial. The other part of the solution was provided by an almost equally brilliant mathematician and experimenter in Holland, Christiaan Huygens. Together, these two extraordinary contemporaries produced a theory of light that in many respects still stands, although the underpinnings of some of their most inspired guesses would not be discovered for two centuries or more.
Newton was a man, essentially, of two works. One was the Principia; the other was called, simply, Opticks. The latter is divided into three books, each of them fascinating in its way.
Part I of Book One begins with a few pages that review what was known about light before Newton began his experiments, many of which were done in 1665-66 when he was still in his early twenties. These Axioms and Definitions are followed by seven Propositions that are among the most revolutionary in the history of science. He shows that the spectrum of white light produced by a prism consists of rays of light of different indexes of refraction and reflection, and that the white light of the Sun consists of these same different rays; and he accurately measures the difference in “refrangibility” and “reflexibility” of the different colors of light in the spectrum (red, orange, yellow, green, blue, indigo, violet). He goes on in Part II to treat colors, about which he knew almost everything that we do. He is particularly acute in discussing our sensation of color. For the rays, to speak properly, he writes, are not colored:
In them there is nothing else from a certain power and disposition to stir up a sensation of this or that colour. For as sound in a bell or musical string, or other sounding body, is nothing but a trembling motion, and in the air nothing but that motion propagated from the object, and in the sensorium ’tis a sense of that motion under the form of sound; so colours in the object are nothing but a disposition to reflect this or that sort of rays more copiously than the rest; in the rays they are nothing but their dispositions to propagate this or that motion into the sensorium, and in the sensorium they are sensations of those motions under the form of colours.
Book Two of the Opticks deals with the colors formed by refraction in and reflection from very thin and very small bodies, such as bubbles or thin plates of glass, or films of water trapped between two prisms. Here Newton not only goes further than anyone before him to explain the nature of color, but he also comes to remarkable conclusions about the bodies themselves with which he has been experimenting. They must, he says, be made up of very small particles between which is something extremely diaphanous or even empty space; and the size of the small particles determines the color of the body. He further shows that bodies can be “extremely porous”—there can be thousands of times as much empty space in them as “solid” matter, yet they may appear to be solid through and through because of the reflections from the particles within the body. “But,” he adds modestly, “what is really their inward frame is not yet known to us.” That inward frame was not discovered and known until the beginning of the twentieth century.
The first two books of the Opticks are interesting and readable. There is little mathematics, for those who detest it; instead there are page after page of fascinating descriptions of Newton’s experiments (which can be duplicated at will), together with his astute conclusions from them. Book Three begins in the same manner, dealing with the bending of light rays around a small object. But this book soon takes another tack and does something that is not to be missed.
When I made the above observations, says Newton, I intended to do them again and more accurately, and I had other experiments in mind to do as well. But I was interrupted, “and cannot now think of taking these things into further consideration. And since I have not finished this part of my design, I shall conclude with proposing only some queries, in order to a further search to be made by others.”
That is modest enough. But what follows is simply amazing. No other man could have written the thirty-one Queries that occupy, with their explanations, the last thirty or so pages of the Opticks. For those thirty-one Queries come very close to defining and outlining the history of physics for the following three hundred years.
Consider just a few of them, as examples. Query 1: Does gravity act upon light at a distance? (Einstein suggested it did; this was confirmed in 1920.) Query 6: Do black bodies absorb light and grow hotter than white ones because they do not reflect the light? (Planck’s experiments with black bodies in the 1890s led to quantum theory.) Queries 8–10: Is heat produced by the rapid agitation of the parts of a body, and if this agitation increases sufficiently, does the body begin to emit light? (The nineteenth century was obsessed with heat; Newton here prefigures many of the most important discoveries of classic thermodynamics.) Query 12 anticipates modern neurophysiology. Query 16 suggests the underlying phenomenon that makes motion pictures possible. Most astonishing of all, Query 30 asks whether gross bodies (we would say matter) and light (we would say energy) are convertible, one into the other. (Einstein, three hundred years later, determined that the answer was yes and that the formula was a simple one: Set as an equation, E = mc2, where E is energy, m is matter, and c2 is the square of the speed of light.) Perhaps equally astonishing, however, is Query 31, which in effect proposes that there must be within bodies a force, different from gravity or magnetic force, that holds the parts of the bodies together and that may even have an effect beyond the bodies. (Here we have the molecular force that could not be accurately measured and defined until the second quarter of the twentieth century.)