Six Lectures on Light | Page 2

Discovery of dark Solar
Rays Invisible Rays the Basis of the Visible Detachment by a Ray-filter
of the Invisible Rays from the Visible Combustion at Dark Foci
Conversion of Heat-rays into Light-rays Calorescence Part played in
Nature by Dark Rays Identity of Light and Radiant Heat Invisible
Images Reflection, Refraction, Plane Polarization, Depolarization,
Circular Polarization, Double Refraction, and Magnetization of Radiant
Heat
LECTURE VI.
Principles of Spectrum Analysis Prismatic Analysis of the Light of
Incandescent Vapours Discontinuous Spectra Spectrum Bands proved
by Bunsen and Kirchhoff to be characteristic of the Vapour Discovery
of Rubidium, Cæsium, and Thallium Relation of Emission to
Absorption The Lines of Fraunhofer Their Explanation by Kirchhoff
Solar Chemistry involved in this Explanation Foucault's Experiment
Principles of Absorption Analogy of Sound and Light Experimental
Demonstration of this Analogy Recent Applications of the
Spectroscope Summary and Conclusion
APPENDIX.
On the Spectra of Polarized Light
Measurement of the Waves of Light
INDEX

ON LIGHT

LECTURE I.
INTRODUCTORY USES OF EXPERIMENT EARLY SCIENTIFIC
NOTIONS SCIENCES OF OBSERVATION KNOWLEDGE OF THE
ANCIENTS REGARDING LIGHT DEFECTS OF THE EYE OUR
INSTRUMENTS RECTILINEAL PROPAGATION OF LIGHT LAW
OF INCIDENCE AND REFLECTION STERILITY OF THE
MIDDLE AGES REFRACTION DISCOVERY OF SNELL PARTIAL
AND TOTAL REFLECTION VELOCITY OF LIGHT ROEMER,
BRADLEY, FOUCAULT, AND FIZEAU PRINCIPLE OF LEAST
ACTION DESCARTES AND THE RAINBOW NEWTON'S
EXPERIMENTS ON THE COMPOSITION OF SOLAR LIGHT HIS
MISTAKE AS REGARDS ACHROMATISM SYNTHESIS OF
WHITE LIGHT YELLOW AND BLUE LIGHTS PRODUCE WHITE
BY THEIR MIXTURE COLOURS OF NATURAL BODIES
ABSORPTION MIXTURE OF PIGMENTS CONTRASTED WITH
MIXTURE OF LIGHTS.
§ 1. Introduction.
Some twelve years ago I published, in England, a little book entitled
the 'Glaciers of the Alps,' and, a couple of years subsequently, a second
book, entitled 'Heat a Mode of Motion.' These volumes were followed
by others, written with equal plainness, and with a similar aim, that aim
being to develop and deepen sympathy between science and the world
outside of science. I agreed with thoughtful men[1] who deemed it
good for neither world to be isolated from the other, or unsympathetic
towards the other, and, to lessen this isolation, at least in one
department of science, I swerved, for a time, from those original
researches which have been the real pursuit and pleasure of my life.
The works here referred to were, for the most part, republished by the
Messrs. Appleton of New York,[2] under the auspices of a man who is
untiring in his efforts to diffuse sound scientific knowledge among the
people of the United States; whose energy, ability, and
single-mindedness, in the prosecution of an arduous task, have won for
him the sympathy and support of many of us in 'the old country.' I
allude to Professor Youmans. Quite as rapidly as in England, the aim of
these works was understood and appreciated in the United States, and

they brought me from this side of the Atlantic innumerable evidences
of good-will. Year after year invitations reached me[3] to visit America,
and last year (1871) I was honoured with a request so cordial, signed by
five-and-twenty names, so distinguished in science, in literature, and in
administrative position, that I at once resolved to respond to it by
braving not only the disquieting oscillations of the Atlantic, but the far
more disquieting ordeal of appearing in person before the people of the
United States.
This invitation, conveyed to me by my accomplished friend Professor
Lesley, of Philadelphia, and preceded by a letter of the same purport
from your scientific Nestor, the celebrated Joseph Henry, of
Washington, desired that I should lecture in some of the principal cities
of the Union. This I agreed to do, though much in the dark as to a
suitable subject. In answer to my inquiries, however, I was given to
understand that a course of lectures, showing the uses of experiment in
the cultivation of Natural Knowledge, would materially promote
scientific education in this country. And though such lectures involved
the selection of weighty and delicate instruments, and their transfer
from place to place, I determined to meet the wishes of my friends, as
far as the time and means at my disposal would allow.
§ 2. _Subject of the Course. Source of Light employed._
Experiments have two great uses--a use in discovery, and a use in
tuition. They were long ago defined as the investigator's language
addressed to Nature, to which she sends intelligible replies. These
replies, however, usually reach the questioner in whispers too feeble for
the public ear. But after the investigator comes the teacher, whose
function it is so to exalt and modify the experiments of his predecessor,
as to render them fit for public presentation. This secondary function I
shall endeavour, in the present instance, to fulfil.
Taking a single department of natural philosophy as my subject, I
propose,