No name is more prominent in optics (or in physics, according to most physicists) than that of Isaac Newton. He may have not gotten everything right in his understanding of the nature of light, says the contemporary physicist, but—for the most part—the Newtonian view of light has remained basically unchanged since the 17th century. If every schoolchild today 'knows' that white light contains all the colours of the spectrum is thanks to Newton, for example. It is also from Newton that just about everyone today ‘knows’ that light is bent by a prism in a beautiful rainbow-like array of colours. It is from Newton that we have inherited the term spectrum.
Newton’s foray into the nature of light began in earnest in 1666 when he “procured” a triangular glass prism and set himself the task of studying the “celebrated phenomena of colours”.
And in order thereto having darkened my chamber, and made a small hole in my window-shuts, to let in a convenient quantity of the Suns light, I placed my Prisme at his entrance, that it might be thereby refracted to the opposite wall. It was at first a very pleasing divertisement, to view the vivid and intense colours produced thereby; but after a while applying my self to consider them more circumspectly, I became surprised to see them in an oblong form; which, according to the received laws of Refraction, I expected should have been circular.
Newton was intrigued by the oblong image and performed a number of experiments to try to find out what caused the elongation of the projected image. Eventually he came to the conclusion that forms the foundation onto which his theory of light and colours rests, namely that “Light it self is a Heterogeneous mixture of differently refrangible Rays”. Newton was driven to this conclusion after he conducted an experiment which he considered crucial to the issue, hence his calling it the experimentum crucis.
I took two boards, and, placing one of them close behind the prism at the window, so that the light must pass through a small hole, made in it for the purpose, and fall on the other board, which I placed at about twelve feet distance, having first made a small hole in it also, for some of the incident light to pass through. Then I placed another prism behind this second board, so that the light trajected through both the boards might pass through that also, and be again refracted before it arrived at the wall. This done, I took the first prism in my hands and turned it to and fro slowly about its axis, so much as to make the several parts of the image, cast on the second board, successively pass through the hole in it, that I might observe to what places on the wall the second prism would refract them. And I saw by the variation of these places that the light, tending to that end of the image towards which the refraction of the first prism was made, did in the second prism suffer a refraction considerably greater than the light tending to the other end. And so the true cause of the length of that image was detected to be no other than that light consists of rays differently refrangible, which, without any respect to a difference in their incidence, were, according to their degrees of refrangibility, transmitted towards divers parts of the wall.
This conclusion seemed to Newton to be reinforced by another experiment, described below in his own words:
Then I suspected whether by any unevenness of the glass or other contingent irregularity these colors might be thus dilated. And to try this I took another prism like the former, and so placed it that the light, passing through them both, might be refracted contrary ways, and so by the latter returned into that course from which the former diverted it. For, by this means, I thought, the regular effects of the first prism would be destroyed by the second prism, but the irregular ones more augmented by the multiplicity of refractions. The event was that the light, which by the first prism was diffused into an oblong form, was by the second reduced into an orbicular one with as much regularity as when it did not all pass through them.
Newton’s view of light and colours were published in the Philosophical Transactions of the Royal Society in February 1672 under the title “A Letter of Mr. Isaac Newton, Professor of the Mathematicks in the University of Cambridge; containing his New Theory about Light and Colors”. It is not a trivial matter to mention, however, that although that paper was said to contain his “theory” of light and colours, Newton firmly believed that
what I shall tell ... is not an Hypothesis but most rigid consequence, not conjectured by barely inferring tis thus because not otherwise or because it satisfies all phaenomena (the Philosophers universall Topick), but evinced by ye mediation of experiments concluding directly & wthout any suspicion of doubt.
Newton’s belief that what he offered was a “most rigid consequence”, rather than a theory (hypothesis), was immediately attacked by Robert Hooke, who was a Fellow of the Royal Society and who had himself a hypothesis about light published in 1664. Following Hooke a number of other people raised their voices against Newton’s new theory, from famous scientists (Huygens) to French Jesuits (Linus and Pardies). Two of those who were questioning Newton’s theory were (still are) considered to be able opponents—intellectually speaking. The others, however, were (still are) perceived as lacking enough intellectual acumen to challenge a mind like Newton’s. This is evident also from the manner in which Newton treated those opponents, although his universal aversion to any sort of criticism was to become legendary. So much so that amid periodical threats that he will completely withdraw from public debates he delayed the publication of his comprehensive work on light and colours until Robert Hooke passed away in 1703. The following year Newton published Opticks: Or, A treatise of the Reflections, Refractions, Inflexions and Colours of Light.
Newton’s theory of light and colours has survived for more than 300 years, and it is an integral part of the foundation of modern physics. Its most important theoretical tenets have remained unchanged, have withstood theoretical and empirical sieges and have emerged victorious. The theory is written in textbooks and taught in universities with such confidence and authority that one would think that no one contests its validity any longer. But if that were the case, one would be wrong. The validity of Newton’s theory of light and colours is still contested today, albeit not by physicists (from what I could gather). It seems to me that Newton’s theory strikes the wrong chord with philosophers, artists, and people involved in chromatic research. Indeed it is the people studying the nature of colours who find Newton’s theory wanting, and it is those people who have brought into question, once again, its validity in recent times. This resurgence of opposition to Newton’s theory irritates physicists, naturally, but I doubt that it causes them to lose any sleep. Philosophers split hairs, kick the ground and make a lot of dust, but that is never enough to sway the physicists from their firm positions behind experiments and mathematics. To kill a theory you need a decisive experiment which categorically shows that the assailed theory is wrong. Nothing less will do—not even a flawless line of reasoning that appears to contradict the theory. And philosophers know that too, hence their reticence to go beyond comments like the following:
Three centuries ago, Newton established the foundation of modern optics and revolutionized the study of colour. These have been lasting achievements, and I have every reason to expect them to endure even as they continue to be modified through the progress of research. Nevertheless, we must realize that the enduring substance of Newton’s discovery is not identical with Newton’s presentation of his experiments and theory; the former is the result of 300 years of critical revision, the latter is fixed in Newton’s writings and in particulars—not always unassailable—of his approach and goals. To us, Newton’s basic theory of light and colours is a good hypothesis, but to Newton it was much more: a fixed and foundational truth that might perhaps be more exactly specified but never refuted. Moreover, we must realize that Newton did not always distinguish carefully what we might call the “physics” underlying color, which is a physical optics of radiation, from the “psychophysics” of color, which tries to correlate the kind of amount of radiation with what is seen and which therefore inevitably draws into the ambit of color science matters of psychology, physiology, and perception. Insofar as he even raised the question, Newton for the most part tried to give the psychophysics of colors a purely physical basis, and thus he really did not provide a comprehensive foundation for the science of color qua color. (Dennis L. Sepper—Goethe contra Newton)
Newton’s theory of light and colours caused quite a stir and triggered immediate opposition right from its outset. This fact is hardly surprising, considering its categorical claims and its radical departure from the view on light and colours predominant at the time. The theory was a revolution against modificationism, and revolutions are never welcomed by any establishment whose beliefs are being attacked. Modificationism was not strictly a theory per se; it was rather an understanding that colours arise from modifications of the white light. This understanding had survived since Aristotle, and although there were a few different views about how colours could be modified from the homogenous white light, no one seems to have questioned, until Newton, the basic tenets of modificationism. It is safe to say that in 1672 everyone was a modificationist of some sort. It is little wonder, then, that with Newton’s radical departure from modificationism some of the modificationists of his day would not lay down their arms easily—even if the new theory seemed to bring forward irrefutable evidence that the traditionalist view of light and colours was wrong.
An interesting observation of those who opposed Newton’s theory in his time was that everyone questioned different aspects of the theory. As Gabor A. Zemplen said in his Ph. D. dissertation (which I found quite insightful):
The comments and criticisms slowly but inescapably arrived. Letters, one after the other, from the various intellectual centres of Western Europe kept questioning the experimental procedure, the propositions, the crucial experiment, the hidden assumptions behind his [Newton’s] view, until the death of Oldenburg and Newton’s deliberate isolation after June 1678 put an end to the correspondence.
From a personal perspective the only significant opposition Newton’s theory faces, even today, is that put forward by Anthony Lucas, a Jesuit Professor of Theology. Many have written about Hooke about Huygens (the men seen as belonging in Newton’s league, intellectually) as worthy opponents to Newton’s theory, but I have been driven (through my own work) to see that Lucas’ arguments were even ‘more worthy’ of consideration. (To my surprise, I am not the only one to believe that—as I’ve come to discover rather recently.) Alas, Newton’s replies to Lucas’ letters leave a lot to be desired, and, frankly, I believe that’s because he saw that Lucas’ arguments were valid and that they were truly testing his theory. In fact Newton did not answer at all the most pertinent of Lucas’ questions, and I have good reasons to contend, even further, that Newton could not answer those questions satisfactorily. On this subject I learned from Zemplen’s dissertation of how others have assessed Lucas’ arguments:
The general assessment of Lucas’s contribution to the debate is not favourable. As Westfall notes: “Lucas’s letters betrayed no particular acumen; the experiments he presented were not well designed” (Westfall 1980: 275). He “espoused the grab-bag method of experimentation”, “his letters manifest a failure to comprehend the very nature of experimental investigation”... Sabra considers Line Gascoigne, and Lucas on a par, and does not analyse the “far less important and not very revealing discussion”... similarly to Hall.
On the other hand Zemplen says a few paragraphs later:
Apart from citing the more common and generally negative valuation it has to be noted that some historians consider Lucas as one of the ablest but little recognised of Newton’s opponents. Already Goethe called him the first clear-headed opponent of Newton. Gruner labelled the dispute “very promising”, and claims that the interesting points were never effectively answered by Newton... Laymon calls Lucas’s arguments “ingenious and bold”... More recently Sepper called Lucas’s critique “sustained and well-planned”...
Personally I found Newton’s debate with Lucas by far the most interesting of all debates Newton had. A careful analysis of their letters is thoroughly enriching even today, I believe, and to that extent I read with great pleasure Zemplen’s account of it. Indeed I was also most surprised by some of the insights and conclusions drawn by Zemplen, for reasons too complex to dwell on now. Nonetheless, in my own analysis of the debate (conducted within the framework of my own understanding of the behaviour of light in prismatic experiments) I came to see Lucas’ arguments from a totally different perspective than that of Zemplen’s (or even from Lucas’ himself). Lucas presented his arguments in three letters to Newton, written in 1676. His arguments were based on a few experiments he devised himself, as well as on his personal observations from replicating Newton’s most important experiments. The experiments Lucas devised were directed at testing Newton’s propositions, and upon conducting them he was convinced that he’d found anomalies which warranted either clear explanations or revisions of the theory. Newton, however, did neither. Zemplen says:
Many of the experiments seem to go against what Newton wrote in his “New Theory”. In case they support his theory rather than overthrow it, it is a prerogative to explain why they are supportive. It might easily be that these experiments can be explained by a slight modification of Newton’s theory or simply by further explanation. But until Newton has not done this, it is reasonable to raise the objections, as, clearly, the experiments go against Newton’s doctrine—as understood by the contemporaries—in its unmodified form. Yet Newton is unconcerned about this. He seems to say that what Lucas is doing is an early stage of inquiry, he has been there, he has done that. He maintains like a good-hearted mentor that first his experiments should be investigated, and seen whether “any of those be demonstrative”. He certainly does not treat Lucas as his equal. He holds that his main point is the refrangibility of light and this point has been demonstrated. He also claims that Lucas’ experimental discourse “has proceeded partly from some misunderstanding of what he writes against, & partly from want of due caution in trying experiments”... Not surprisingly, then, there is no detailed answer to any of the objections Lucas raised. But there are no arguments convincing us that Newton can incorporate Lucas’s observations into his theory, only authoritative claims...
From my perspective the most important of Lucas’ observations and arguments are those related to the amount of the spectral elongation and to the nature of the spectral colours he observed directly through the prism. I should perhaps mention at this point that all prismatic experiments have been classified into two types: the first type have been called objective experiments (the basic prismatic experiment—where the spectrum emerging from a single prism is cast onto a screen—and the crucial experiment belong to this type); the second type (where the spectral colours are observed directly through the prism) have been dubbed subjective experiments. The difference between the two types of experiments may seem self-evident and fully warranted, at a first glance, but the reasons behind this classification are highly debatable, to my mind. Nevertheless, I will stop short of getting into what would unavoidably be a long and contentious issue, and I will thus comply with the accepted view.
The first experiment I found important in Lucas’ letters was the following:
I fastened a very white Paper-circle (about an inch in diameter) upon my Window-shuts; and beholding it through my prism, I found a Coloured image painted thereby upon my Retina, answerable in almost all respects to the former of the Sun-beams upon the Wall, especially when the Paper-circle was indifferently well illuminated. This Image indeed appeared contrary to the former as to the scituation of Colours, that is, the Scarlet appearing above, the Violet below, though but faint.
This experiment is the subjective version of the basic prismatic experiment described by Newton in the letter to the Royal Society that contained his “New Theory about light and colours”. The objective experiment was the one where Newton let a narrow beam of sunlight pass through a triangular prism and projected the emerging image of the spectrum on the opposite wall. Newton left us a drawing of that experiment.
Now, the only substantial difference between the objective version of this experiment (seen above) and its subjective counterpart is that Lucas substituted the wall with his retina (to use a somewhat poetic language, like he did). In other words Lucas looked through the prism at his paper-circle fastened on his window-shuts. The image of the spectrum Lucas observed was in all respects identical with the image projected by Newton on the wall, with the exception that the spectrum Lucas saw was an inverted copy of Newton’s spectrum. But Newton knew that already. Years before publishing his theory Newton had written in a notebook: “Prismaticall colours appeare in the eye in a contrary order”.
So why do I find Lucas’ observation interesting? For a number of very important reasons. Firstly, because in spite of being aware of this observational fact, Newton did never provide an explanation for it. Secondly, because Lucas did offer an explanation (which can be easily shown to be wrong, even naive), but Newton chose to make no remark at all on the subject. Thirdly, because I have challenged physicists around the world—for years now—to provide an explanation, without success. (Only one offered some ‘explanation’, with a validity almost on a par with Lucas’.) Lastly, but most importantly, because I know the real reason for that observational fact.
The order of the spectral colours is different in subjective prismatic experiments, and that is held to be the only difference between the two types of experiments. Despite this difference, as far as I know no one has paid what I consider to be vital attention to it. Lucas, as I said before, offered an explanation for the inversion of the spectral colours when one looks through a prism:
But this I was not surprized at, having observ'd upon dissecting the eye, that objects are painted on the Retina after a contrary posture to what they appear to Sight. Having thus rendred the Coloured image much more tractable than formerly it was, I conceived good hopes of some further discovery in the point mentioned.
Newton had begun his investigation of light
and colours as a direct result of his observing that the spectrum
cast on his wall (from a circular source) was in the shape of an
extended oblong, instead of an orbicular one—which should have been
the case if “the received laws of Refraction” were correct. This
observation was for Newton the smoking gun in the mystery of light
and prismatic colours. The oblong shape of the spectrum in a setup
where the angles of the incident and emergent beams of light were
the same required an explanation—which the known laws of
refraction and the modificationist understanding of light could not
provide. Newton was determined to find out what caused that spectral
elongation, and he came to an answer that satisfied him upon
conducting his experimentum crucis. The oblong shape,
described in his theory (and clearly drawn in the picture I showed
earlier), was about five times longer than wide, which was—in
Newton’s words, “a disproportion so extravagant, that it excited
me to a more than ordinary curiosity of examining from whence it
might proceed”.
Lucas had also obtained an oblong spectrum
when he repeated Newton’s basic experiment, but he maintained that
his spectrum was, at the most, three and a half times longer than
wide. In fact Lucas said that he performed the experiment many times
and in many conditions (weather wise), but that he “never yet have
found the excess above thrice the diameter, or at most 3 ½, while
the refractions on both sides of the Prism were equal”. In his
reply to Lucas Newton paid special attention to the ratio between the
breadth of the spectrum and its length, maintaining that his
observation of the five to one ratio is the right average. He does
not contest Lucas’ ratio, but he puts it down to differences
between his prisms and Lucas’. He then presents a few tables with
measurements he had compiled by using prisms with different
refractive angles. The ratios in those tables vary from about four to
five and a half to one. All in all, he explains that the
difference between their observations are a consequence of their
using prisms with different refractive angles and different
refractive powers. Ultimately, Lucas’ smaller ratio does not carry
any theoretical implications—it is simply the result of a different
apparatus.
From a personal perspective, Newton’s
elaboration on the subject of spectral elongation is much more
important than his quantitative assessment. That’s because the
elongation of the spectrum is the foundation onto which the Newtonian
theory of light and colours rests. More precisely, the current
theoretical understanding of light and its behaviour in prismatic
experiments is strictly dependent on Newton’s doctrine that white
light is formed by different lights which are independently and
unequally refracted in a prism. There is no other theory out there,
it has been stated, that can explain the fact that the spectrum from
a circular source has an oblong shape better than Newton’s theory.
(In fact there isn’t any theory out there contesting Newton’s
theory. There might be a re-emergence of modificationism in some
circles today, but a theory per se, no.) Nevertheless,
Newton’s theory has created many problems as well, especially in
its description of colours and in its ability to explain the
observations of subjective prismatic experiments. It was within this
context that in my own investigation of the prismatic phenomena I
have discovered that there is another way that can render a
circular source of light into an oblong spectrum. It was imperative,
then, that that way—while different to Newton’s—conforms
and explains all things Newton’s theory explains plus
those things Newton’s theory does not explain. With
this uncompromising condition in mind, I found that my way can
beautifully explain the facts Newton mentioned in his first reply to
Lucas—namely that the amount of elongation of the spectrum depends
on the refracting angle of the prism and its refractive power. Unlike
in the Newtonian theory, however, through my way the oblong shape of
the spectrum occurs without the need of unequal refractions for
the different lights that make up the so-called white light. And
this is not a trivial matter. I will discuss later what causes the
elongation of the spectrum in my understanding, as well as what
implications my understanding carry.
Apart from observing a reversal of the
spectral colours and a different amount of elongation Lucas describes
in his first letter a number of experiments which appear to
contradict Newton’s doctrine of specific refractions for colours.
The experiments described are mainly subjective experiments, and, in
essence, they were to form later the basis of Goethe’s rejection of
Newton’s theory. One of those experiments is important and
insightful:
I
placed the Paper-circle anew, so as the one half b was fastened to
the Shuts, the other semicircle a being exposed to the open Air.
Whereupon the semicircle a became bordered with Violet above, Scarlet
below; but the other semicircle b quite contrary. Hence I make the
following Inferences. First,
That not only the Light reflected from the Paper-circle, but also
from the ambient Air, hath great influence upon the Coloured image,
especially as to the Violet and Scarlet colours. Whence perchance it
will not hereafter seem strange, that the coloured Spectrum on the
Wall is so long, but only that the breadth is not greater. Secondly,
Were there a more luminous body behind the Sun, we should in all
likelyhood have the colours of the Spectrum in a contrary scituation
to what they appear in at present: Whence (thirdly) it seems to
follow, that the present scituation and order of Colours, ariseth not
from any intrinsecal property of refrangibility (as maintained by Mr.
Newton) but from contingent and extrinsecal circumstances of
neighbouring objects. For accordingly as the body behind the
Paper-circle was more or less illuminated than the Circle it self,
all the several Colours changed their scituation.
Here again Lucas observes a reversal of
colours, but this time his inference (the second one, that the nature
of the background creates the reversal) is correct. Newton did not
cover this situation in his theory, but it can certainly incorporate
it without difficulty. Lucas’ third inference is the most
interesting, however. Not because it is correct in its assertion that
the order of colours “ariseth not from any intrinsecal property of
refrangibility... but from contingent and extrinsecal
circumstances”—for that’s not necessarily the cause. Zemplen,
for instance, writes in his dissertation about this issue the
following:
In his reply ...Newton maintains that
Lucas’s mistake arises from not considering:
“yt
when equal lights...are conterminate, ye colours arise from ye
greater light, ye less light being in respect of ye greater but as a
shadow...”
This means that even viewing the
same light source through the prism gives rise to different effects
depending on the surroundings. The order of
colours in the image, i.e. the nature of the spectral image is
dependent not only on light and the prism, but on the
relative luminosity of the image/light. White as a
source of light (with its rays differently coloured) does not
determine the spectral image, only if the surroundings are less
white, and not whiter.
Newton’s explanation sounds plausible
enough, and Lucas’ inferences have done nothing to shake the basis
of Newton’s theory, but the experiment he described, coupled with
the observation of the colours displayed, should have been paid much
more attention than it has been. That’s because although it is
plausibly explainable in Newtonian terms, it nevertheless carries
within itself some inconspicuous facts whose implications go far
beyond Lucas’ inferences and the conventional explanation of them.
To examine the facts I’m talking about I will replicate, in
principle, Lucas’ experiment in the figure below. The experiment
consists of a circle placed on two different backgrounds—one
background is more luminous than the circle, the other is darker. To
simplify the experiment (without changing the relevant factors) I
placed a grey circle on a background half white half black.
If you now look at the figure through a
triangular prism oriented with the refractive angle (the vertex)
pointing upwards you will see the reversal of the spectral colours
Lucas described. In effect, you will see the upper half of the grey
circle bordered (on the part over the white background) by red and
yellow, and on the part over the black background by violet, blue and
cyan. The bottom half of the circle will also be split into two
coloured quarters, this time, however, the coloured groups being
reversed: violet- blue- cyan on the white background and the
red-yellow combination on the black background. This reversal of the
spectral colours was explained by Newton to arise from the fact that
in such cases the colours observed are predominantly those produced
by the “greater lights”, which ‘overshadow’ the colours
produced by the circle itself. This is a plausible explanation, and
it can indeed account for the observed colours. Newton, however, and
especially his followers failed to pursue this explanation further,
which would have led them to a full understanding of the phenomenon,
which—in turn—would have enabled them to predict with
absolute certainty what colours appear where, and why. Even
Goethe should have arrived at that understanding, considering the
great number and the attention he paid to subjective experiments. In
fact it is hard to believe that more than three hundred years later
no one can predict, with absolute certainty, what colours
are observed in all subjective experiments. But I can tell
you, with absolute certainty, that by the end of reading these pages
you will be able to.
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