Opticks [Paperback]

As this review will appear on multiple editions of Opticks, please select carefully which you purchase. The present edition of this classic work in the history of science is a printing of an edition which was published in 1931 by Messrs G. Bell and Sons of London with a foreword by Albert Einstein and an introduction by E.T. Whittaker. To this wealth of previous introductory material the present edition adds an illuminating preface by I. Bernard Cohen. In an original contribution to the history of science he sets forth the influence that Newton's Opticks, or a treatise of the reflections, refractions, inflections and colours of light, exerted on other sciences during the eighteenth and nineteenth centuries. For physical theory Newton's Opticks presented a corpuscular theory of light which dominated physical thought during the eighteenth century. But Cohen shows that Thomas Young, who performed basic experiments for the wave theory of light, held that his views merely developed those of Newton, whose work Young cited extensively. Cohen also refers to the researches of Marjorie Hope Nicolson, to whom the present edition is dedicated in recognition of her revelation of the influence of the Opticks on the literary imagination in the eighteenth century.

The Opticks is divided into the First Book, parts I and II; the Second Book, parts I, II, III, IV; the Third Book, part I. Newton began part I of the First Book with the statement that his design was not to explain the properties of light by hypotheses, but to propose and prove them by reason and experiments. The First Book contains Newton's theory of colors and describes in detail how he varied conditions in order to demonstrate that white light is composed of invariable components. The Second Book describes observations concerning the reflections, refractions and colors of thin transparent bodies. For the description of phenomena of colors of thin films, Newton introduced the concept of "Fits of easy Reflection" for the return of the disposition of any ray of light to be reflected, and also introduced the concept of "Fits of easy Transmission" for the return of its disposition to be transmitted. The interval of its "Fits" is related to the wave length of subsequent wave theory by which the phenomena observed by Newton can be explained readily in terms of interference. The Third Book reports observations concerning the inflections of rays of light, and the colors made thereby. Since Newton did not observe that light is bent into the shadow of a straight edge, a wave phenomenon which is difficult to observe on account of the short wave length of light, he stated his adherence to a corpuscular theory of light. From the phenomenon of double refraction, Newton inferred that the rays of light have sides. The corpuscular theory of light conformed to the general physical theory of atomism which Newton set forth as follows: "It seems probable to me, that God in the Beginning form'd Matter in solid, massy, hard, impenetrable, moveable particles..." "That Nature may be lasting, the Changes of corporeal Things are to be placed only in various Separations and new Associations and Motions of these permanent Particles." (p. 400)

Newton's work on optics is of permanent value for its description of original experiments by which new phenomena were discovered, on account of the fundamental nature of the phenomena and also because the experiments admirably illustrate the experimental method of control and variation of conditions of a phenomenon. 20th-Century interest in the Opticks was stimulated by Newton's supplementation of the corpuscular theory by the idea of vibrations in a medium. Newton conjectured that when the rays of light strike a solid body the ether is set in vibration. This combination of corpuscular and wave theory, however, cannot be claimed to have anticipated the dualism of corpuscle and wave aspects of matter and radiation which characterizes quantum theory.

Newton outlined his method of procedure in the next to the last paragraph of the third book of the Opticks: "As in Mathematicks, so in Natural Philosophy, the investigation of difficult things by the Method of Analysis, ought ever to precede the Method of Composition. This Analysis consists in making Experiments and Observations, and in drawing general Conclusions from them by Induction, and admitting of no Objection against the Conclusions, but such as are taken from Experiments, or other certain Truths. For Hypotheses are not to be regarded in experimental philosophy." In the terminology of contemporary theory, Newton sought operational definitions of physical concepts. He rejected the occult qualities of the Aristotelians as "Qualities unknown to us and uncapable of being discovered and made manifest." (p. 401)

It may be argued that in the Opticks Newton reserved his hypotheses for the Queries. To this one may reply that he expressly stated that the Queries concerned problems of the properties of light on which he did not have time to make further observations and studies. To quote: "And since I have not finished this part of my Design, I shall conclude with proposing only some Queries, in order to a farther search to be made by others." (p. 338) In the very last paragraph of the Opticks Newton stated that in the first two books he proceeded by Analysis. "In this Third Book," he added, "I have only begun the Analysis of what remains to be discovered about Light and its Effects upon the Frame of Nature, hinting several things about it and leaving the Hints to be examined and improved by the further Experiments and Observations of such as are inquisitive."

Newton declared that the business of natural philosophy is to argue from phenomena without feigning hypotheses, and to deduce causes from effects, till we come to the very first cause, which certainly is not mechanical. (p. 369) In this procedure of finding causes he used his constructive imagination in the Principia as well as in the Opticks.

In an appreciative foreword, Albert Einstein declares that in one person Newton combined the experimenter, the theorist, the mechanic and, not least, the artist in exposition. I heartily subscribes to the further statement of the foreword: "this new edition of his work on optics is...to be welcomed with warmest thanks, because it alone can afford us the enjoyment of a look at the personal activity of this unique man."

There are two main arguments for a corpuscular view of light:

(1) Light consists of rays of inherent and inalterable dispositions (as regards colour, refrangibility, etc.). This is argued for throughout, but see esp. the classic prism experiments in props. I and II. Wave theorists, on the contrary, base their explanations on modifications of rays.

(2) The law of refraction "may be demonstrated upon this Supposition. That Bodies refract Light by acting upon its Rays in Lines perpendicular to their Surfaces" (p. 79). Consider what happens as the ray passes through the strip from y=c to the surface boundary at y=0. Newton states the lemma that the vertical velocity v_2 at y=0 will be determined by the initial vertical velocity v_1 at y=c and the would-be vertical velocity v_0 at y=0 if v_1 had been 0, as follows: v_2^2=v_1^2+v_0^2. Newton omits the proof as being too easy; it may be supplied as follows. Think of the v's as functions of y and differentiate. Both sides vill be of the form 2v(dv/dy) = 2(dy/dt)(dv/dy) = 2(dv/dt) = 2a = proportional to F, which is equal at equal y's. Thus since the lemma holds for c=0 and the derivatives are equal it holds generally. Though Newton emphasises that he has not assumed anything about the nature of light, we see that this proof makes most sense from a corpuscular point of view since it in effect appeals to F=ma. A further side effect of this proof is that it implies that light speeds up when it is refracted towards the normal, which implies that light is slowest in vacuum and fastest in dense materials.

(1) and (2) are elegantly combined if rays of different colours consist of particles of different sizes.

Further arguments against the wave theory (Query 28): light does not "bend into the Shadow"; it cannot explain "the unusual Refraction of Island Crystal"; it needs two aethers to explain the "fits" in the next paragraph; "against filling the Heavens with fluid Mediums, unless they be exceedingly rare, a great Objection arises from the regular and very lasting Motions of the Planets ... for thence it is manifest, that the Heavens are void of all sensible Resistance."

Nevertheless, waves are needed to explain interference patterns and the fact that light is in alternating "fits of easy Reflexion and easy Transmission" (p. 281). This can be done as follows (Query 18). When light goes from one medium to another it strikes the aether, creating waves like ripples on a pond. These ripples travel faster than the rays and "by endeavouring to expand itself" (p. 352) the aether is thus alternately pushing and pulling on the rays, causing the fits. The existence on the aether is suggested by the fact that thermometers behave the same in vacuum as in air (Query 18). Since light and heat are capable of generating each other (Queries 8-10), it is plausible that this aether is the same as that of light.

The other major theme in the book is colour theory. This is all very good, but it is not very exciting since these ideas are so commonplace today. An interesting exception is Query 14 where it is suggested that harmonious colour combinations are due to harmonious combinations of frequencies, as in music.

Here is an example of a classical document from the history of physics. An interesting text from one of the greatest physicists of all time, Isaac Newton.