Department of History
University of California, Irvine
Instructor:    Dr. Barbara J. Becker

Lecture 12.  Light and Enlightenment.


Newton's Optical Studies


No color will result from a mixture of pure black and white.  If that happened, then pictures drawn with ink would appear colored, printed pages would appear colored at a distance, and the edges of shadows would be colored.  Therefore, colors cannot arise from the reflection of light or of shadows mixed with light.

See if two prisms--one casting blue upon the other's red--do not make a white.

The rays which make blue are refracted more than the rays which make red.  This can be seen in the following experiment:

On a black peice of paper I drew a line opq, whereof one halfe op was a good blew ye other pq a good deepe red (chosen by Prob of Colours).  And looking on it through ye Prisme adf, it appeared broken in two betwixt ye colours, as at rst, ye blew parte rs being nearer ye vertex ab of ye Prisme yn ye red parte st.  Soe yt blew rays suffer a greater refraction yn red ones.  Note  [I call those blew or red rays &tc. wch make ye Phantome of such colours.  The same experiment may bee tryed wth a thred of two colours held against ye darke.


Prismatical colors appear in the eye in the opposite order to that which they fall on the paper.

If the sun shines on a prism, some of the rays are refracted through the prism and make colors on the wall, while other rays are reflected out another side of the prism.  The reflected rays are only white and show no colors.

If you turn the prism, so that the colorful rays leave the prism at a sharper and sharper angle, the blue color will suddenly vanish from the colorful array on the wall and will, instead, join with the reflected rays.  The reflected rays will now appear a little bluish.

If you turn the prism even more, the red color will vanish too from the colorful array, and join the reflected rays, making the bluish color turn white again.

If three or more prisms (A, B, and C) are held in the sun so that the red color of prism B falls upon the green or yellow color of prism A, and the red color of prism C falls on the green or yellow color of prism B, then when that colored light that falls on a paper screen it will appear red at one end and blue at the other, but in between, where all the colors of the different prisms are blended together, it will appear white.

Thin flakes of mica, bubbles which children make of soap and water, water wiped very thin on glass, glass blown very thin, etc exhibit the phenomena of the colored circles.


Title page of Opticks (1704) by Isaac Newton

Sir Isaac Newton himself notwithstanding the great advantage which he derived from patient thinking, indulged bold and eccentric thoughts of which his Queries at the end of his book of Optics are a sufficient evidence.
--Joseph Priestley

The Queries

When I made the foregoing Observations, I design'd to repeat most of them with more care and exactness, and to make some new ones for determining the manner how the Rays of Light are bent in their passage by Bodies, for making the Fringes of Colours with the dark lines between them.  But I was then interrupted, and cannot now think of taking these things into farther Consideration.  And since I have not finish'd this part of my Design, I shall conclude with proposing only some Queries, in order to a farther search to be made by others.

Query 1
Do not Bodies act upon Light at a distance, and by their action bend its Rays; and is not this action strongest at the least distance?

Query 2
Do not the Rays which differ in Refrangibility differ also in Flexibility; and are they not by their different Inflexions separated from one another?...

Query 5
Do not Bodies and Light act mutually upon one another?...

Query 8
Do not all fix'd Bodies, when heated beyond a certain degree, emit Light and shine; and is not this Emission perform'd by the vibrating motions of their parts?...

Query 13
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?...

Query 26
Have not the Rays of Light several sides, endued with several original Properties?

Query 27
Are not all Hypotheses erroneous which have hitherto been invented for explaining the Phenomena of Light, by new Modifications of the Rays?  For those Phenomena depend not upon new Modifications, as has been supposed, but upon the original and unchangeable Properties of the Rays.

Query 28
Are not all Hypotheses erroneous, in which Light is supposed to consist in Pression or Motion, propagated through a fluid Medium?

Query 29
Are not the Rays of Light very small Bodies emitted from shining Substances?

Query 30
Are not gross Bodies and Light convertible into one another, and may not Bodies receive much of their Activity from the Particles of Light which enter their Composition?

Query 31
Have not the small Particles of Bodies certain Powers, Virtues, or Forces, by which they act at a distance?

According to Newton, Reason is immutable and universal--
  • all have the capacity to exercise it
  • all phenomena (animate and inanimate; individual and collective) are legitimate subjects of reasoned investigation
Reason has one method:  Newton's method--
  • observe specifics (analysis)
  • draw up general principles (synthesis)
  • deduce new phenomena by identifying analogous effects and assigning analogous causes
Newton's Legacy:  Science in the Age of Reason
To the illustrious man, Isaac Newton, and this [Principia] his work, done in fields of Mathematics and Physics, a signal distinction of our Time and Race
...The inmost places of the heavens, now gained,
Break into view, nor longer hidden is
The force that turns the farthest orb.  The sun
Exalted on his throne bids all things tend
Toward him by inclination and descent,
Nor suffers that the courses of the stars
Straight, as through the boundless void they move,
But with himself as centre speeds them on
In motionless ellipses.  Now we know
The sharply veering ways of comets, once
A source of dread, nor longer do we quail
Beneath appearances of bearded stars.


Then ye who now on heavenly nectar fare,
Come celebrate with me in song the name
Of Newton, to the Muses dear; for he
Unlocked the hidden treasuries of Truth:
So richly through his mind had Phoebus cast
The radiance of his own divinity.
Nearer the gods no mortal may approach.

    --Edmund Halley (1656-1742)
       Preface to Principia (1687)

I have first considered mankind; and the result of my thoughts has been, that amidst such an infinite diversity of laws and manners, they were not solely conducted by the caprice of fancy.

I have laid down the first principles, and have found that the particular cases follow naturally from them; that the histories of all nations are only consequences of them; and that every particular law is connected with another law, or depends on some other of a more general extent....

I have not drawn my principles from my prejudices, but from the nature of things.

--Charles Montesquieu (1689-1755)
    Preface to Spirit of the Laws (1748)

Our century is called ... the century of philosophy par excellence...

The discovery and application of a new method of philosophizing, the kind of enthusiasm which accompanies discoveries, a certain exaltation of ideas which the spectacle of the universe produces in us--all these causes have brought about a lively fermentation of minds, spreading through nature in all directions like a river which has burst its dams.

--Jean Lerond d'Alembert (1717-1783)
   Elements of Philosophy (1759)

The most ancient of all societies, and the only one that is natural, is the family:  and even so the children remain attached to the father only so long as they need him for their preservation.  As soon as this need ceases, the natural bond is dissolved....

This common liberty results from the nature of man.  His first law is to provide for his own preservation, his first cares are those which he owes to himself; and, as soon as he reaches years of discretion, he is the sole judge of the proper means of preserving himself, and consequently becomes his own master.

--Jean-Jacques Rousseau (1712-1778)
   The Social Contract (1762)

--Immanuel Kant (1724-1804)
   What is Enlightenment? (1784)

Dare to know!  "Have the courage to use your own reason!"--THAT is the motto of the enlightenment. 

Laziness and cowardice are the reasons why such a large part of mankind gladly remain minors all their lives, long after nature has freed them from external guidance.  They are the reasons why it is so easy for others to set themselves up as guardians. 

It is so comfortable to be a minor.  If I have a book that thinks for me, a pastor who acts as my conscience, a physician who prescribes my diet, and so on--then I have no need to exert myself.  I have no need to think, if only I can pay; others will take care of that disagreeable business for me....

This enlightenment requires nothing but freedom ... to make public use of one's reason in all matters.... 

When we ask, Are we now living in an enlightened age? the answer is, No, but we live in an age of enlightenment.

The Newtonians
David Gregory (1659-1708)
  • first to teach pupils on the basis of the Newtonian philosophy
John Keill (1671-1721)
  • studied at Edinburgh under Gregory
  • gave first lectures in which Newton's laws were derived experimentally
  • interested in short-range forces between small particles as described in Query 31
  • elaborated on problems he felt that Newton had been unable to pursue in his lifetime

John Desaguliers (1683-1744)

  • inherited Keill's teaching position
  • offered a course of demonstration lectures in natural philosophy
  • rival of Francis Hauksbee (c.1666-1713), who succeeded Robert Hooke as Curator of Experiments for Royal Society
  • on Hauksbee's death, became new Curator of Experiments and chief promoter of Newtonian views

Willem s'Gravesande (1688-1742)

  • leader of Newtonian philosophy in Holland
  • visited England; did experiments with Desaguliers
  • wrote textbook:  Elements of Natural Philosophy

Voltaire (1694-1778)

  • proponent and popularizer of Newtonian ideas

This famous Newton, this destroyer of the Cartesian system, died in March, anno 1727.  His countrymen honoured him in his lifetime, and interred him as though he had been a king who had made his people happy....

--from Letter XIV, Letters on the English (1778)

Sir Isaac Newton has demonstrated to the eye, by the bare assistance of the prism, that light is a composition of coloured rays, which, being united, form white colour.  A single ray is by him divided into seven, which all fall upon a piece of linen, or a sheet of white paper, in their order, one above the other, and at unequal distances.  The first is red, the second orange, the third yellow, the fourth green, the fifth blue, the sixth indigo, the seventh a violet-purple.  Each of these rays, transmitted afterwards by a hundred other prisms, will never change the colour it bears; in like manner, as gold, when completely purged from its dross, will never change afterwards in the crucible.  As a superabundant proof that each of these elementary rays has inherently in itself that which forms its colour to the eye, take a small piece of yellow wood, for instance, and set it in the ray of a red colour; this wood will instantly be tinged red.  But set it in the ray of a green colour, it assumes a green colour, and so of all the rest.

--from Letter XVI, Letters on the English (1778)

Émilie du Châtelet (1706-1749)

  • translated Principia into French
  • wrote Institutions Physiques

Putti [angelic children] play badminton in an illustration of the laws of physics at work in everyday activities, from Émilie du Châtelet's Institutions Physiques.

Benjamin Franklin (1706-1790)

  • applied Newton's method to study of electricity
  • viewed electricity's behavior as analogous to that of gravity
Go to:
  • The Regulæ [Rules for the Direction of the Mind] (1628) by René Descartes (1596-1650);
    • Philosophia Naturalis Principia Mathematica [Mathematical Principles of Natural Philosophy] (1689);
    • Opticks (1704); and
Weekly Readings
Lecture Notes