sense is proportioned, the perception and knowledge of which is of
the greatest use to mankind. The same is the groundwork of
philosophy; for, though all sorts and degrees are equally the object
of philosophical speculation, yet it is from those which are
proportioned to sense that a philosopher must set out in his
inquiries, ascending or descending afterwards as his pursuits may
require. He does well indeed to take his views from many points of
sight, and supply the defects of sense by a well-regulated
imagination; nor is he to be confined by any limit in space or time;
but, as his knowledge of Nature is founded on the observation of
sensible things, he must begin with these, and must often return to
them to examine his progress by them. Here is his secure hold: and
as he sets out from thence, so if he likewise trace not often his
steps backwards with caution, he will be in hazard of losing his way
in the labyrinths of Nature.'--(_Maclaurin: An Account of Sir I.
Newton's Philosophical Discoveries. Written 1728; second edition_,
1750; pp. 18, 19.)
]
[Footnote 10: I do not wish to encumber the conception here with the
details of the motion, but I may draw attention to the beautiful model
of Prof. Lyman, wherein waves are shown to be produced by the
_circular_ motion of the particles. This, as proved by the brothers
Weber, is the real motion in the case of water-waves.]
[Footnote 11: Copied from Weber's _Wellenlehre_.]
[Footnote 12: See _Lectures on Sound_, 1st and 2nd ed., Lecture VII.;
and 3rd ed., Chap. VIII. Longmans.]
[Footnote 13: _Boyle's Works_, Birch's edition, p. 675.]
[Footnote 14: Page 743.]
[Footnote 15: The beautiful plumes produced by water-crystallization
have been successfully photographed by Professor Lockett.]
[Footnote 16: In a little volume entitled 'Forms of Water,' I have
mentioned that cold iron floats upon molten iron. In company with my
friend Sir William Armstrong, I had repeated opportunities of
witnessing this fact in his works at Elswick, 1863. Faraday, I
remember, spoke to me subsequently of the perfection of iron castings
as probably due to the swelling of the metal on solidification. Beyond
this, I have given the subject no special attention; and I know that
many intelligent iron-founders doubt the fact of expansion. It is
quite possible that the solid floats because it is not _wetted_ by the
molten iron, its volume being virtually augmented by capillary
repulsion. Certain flies walk freely upon water in virtue of an action
of this kind. With bismuth, however, it is easy to burst iron bottles
by the force of solidification.]
[Footnote 17: This beautiful law is usually thus expressed: _The index
of refraction of any substance is the tangent of its polarizing
angle_. With the aid of this law and an apparatus similar to that
figured at page 15, we can readily determine the index of refraction
of any liquid. The refracted and reflected beams being visible, they
can readily be caused to inclose a right angle. The polarizing angle
of the liquid may be thus found with the sharpest precision. It is
then only necessary to seek out its natural tangent to obtain the
index of refraction.]
[Footnote 18: Whewell.]
[Footnote 19: Removed from us since these words were written.]
[Footnote 20: The only essay known to me on the Undulatory Theory,
from the pen of an American writer, is an excellent one by President
Barnard, published in the Smithsonian Report for 1862.]
[Footnote 21: _Boyle's Works_, Birch's edition, vol. i. pp, 729 and
730.]
[Footnote 22: _Werke_, B. xxix. p. 24.]
[Footnote 23: Defined in Lecture I.]
[Footnote 24: This circumstance ought not to be lost sight of in the
examination of compound spectra. Other similar instances might be
cited.]
[Footnote 25: The dark band produced when the sodium is placed within
the lamp was observed on the same occasion. Then was also observed for
the first time the magnificent blue band of lithium which the Bunsen's
flame fails to bring out.]
[Footnote 26: New York: for more than a decade no such weather had
been experienced. The snow was so deep that the ordinary means of
locomotion were for a time suspended.]
[Footnote 27: 'Il faut reconnaître que parmi les peuples civilisés de
nos jours il en est pen chez qui les hautes sciences aient fait moins
de progrès qu'aux États-Unis, ou qui aient fourni moins de grands
artistes, de poëtes illustres et de célèbres écrivains.' (_De la
Démocratie en Amérique_, etc. tome ii. p. 36.)]
[Footnote 28: At these points the two rectangular vibrations into
which the original polarized ray is resolved by the plates of gypsum,
act upon each other like the two rectangular impulses imparted to our
pendulum in Lecture IV., one being given when the pendulum is at the
limit of its swing. Vibration is thus converted into rotation.]
[Footnote 29: The millimeter is about 1/25th of an inch.]
INDEX.
Absorption, principles of, 199
Airy, Sir George, severity and conclusiveness of his proofs, 209
Alhazen, his inquiry respecting light, 14, 207
Analyzer, polarizer and, 127
----recompounding of the two systems of waves by the analyzer, 129
Ångström, his paper on spectrum analysis, 202
Arago, François, and Dr. Young, 50
----his discoveries respecting light, 208
Atomic polarity, 93-96
Bacon, Roger, his inquiry respecting light, 14, 207
Bartholinus, Erasmus, on Iceland spar, 112
Bérard on polarization of heat, 180
Blackness, meaning of, 32
Boyle, Robert, his observations on colours, 65, 66
----his remarks on fluorescence, 163, 164
Bradley, James, discovers the aberration of light, 21, 22
Brewster, Sir David, his chief objection to the undulatory theory of
light, 47
Brewster, Sir David, his discovery in biaxal crystals, 209
Brougham, Mr. (afterwards Lord), ridicules Dr. T. Young's
speculations, 50, 51
Cæsium, discovery of, 193
Calorescence, 174
Clouds, actinic, 152-154
----polarization of, 155
Colours of thin plates, 64
----Boyle's observations on, 65, 66
----Hooke on the colours of thin plates, 67
----of striated surfaces, 89, 90
Comet of 1680, Newton's estimate of the temperature of, 168
Crookes, Mr., his discovery of thallium, 193
Crystals, action of, upon light, 98
----built by polar force, 98
----illustrations of crystallization, 99
----architecture of, considered as an introduction to their action upon
light, 98
----bearings of crystallization upon optical phenomena, 106
Crystals, rings surrounding the axes of, uniaxal and biaxal, 145
Cuvier on ardour for knowledge, 220
De Tocqueville, writings of, 215, 222, 223
Descartes, his explanation of the rainbow, 24, 25
----his ideas respecting the transmission of light, 43
----his notion of light, 207
Diamond, ignition of a, in oxygen, 169
Diathermancy, 173
Diffraction of light, phenomena of, 78
----bands, 78, 79
----explanation of, 80
----colours produced by, 89
Dollond, his experiments on achromatism, 28
Draper, Dr., his investigation on heat, 172
Drummond light, spectrum of, 195
Earth, daily orbit of, 74
Electric beam, heat of the, 168
Electricity, discoveries in, 217, 218
Emission theory of light, bases of the, 45
----Newton espouses the theory, and the results of this espousal, 77
Ether, Huyghens and Euler advocate and defend the conception of an, 48, 58
----objected to by Newton, 58
Euler espouses and defends the conception of an ether, 48, 58
Eusebius on the natural philosophers of his time, 13
Expansion by cold, 104
Experiment, uses of, 3
Eye, the, its imperfections, grown for ages towards perfection, 8
----imperfect achromatism of the, 29, _note_
Faraday, Michael, his discovery of magneto-electricity, 218
'Fits,' theory of, 73
----its explanation of Newton's rings, 74
----overthrow of the theory, 77
Fizeau determines the velocity of light, 22
Fluorescence, Stokes's discovery of, 161
----the name, 174
Forbes, Professor, polarizes and depolarizes heat, 180
Foucault, determines the velocity of light, 22
----his experiments on absorption, 197, 198
Fraunhofer, his theoretical calculations respecting diffraction, 87
----his lines, 193
------their explanation by Kirchhoff, 193
Fresnel, and Dr. Young, 50
----his theoretical calculations respecting diffraction, 87
----his mathematical abilities and immortal name, 210
Goethe on fluorescence, 165
Gravitation, origin of the notion of the attraction of, 92
----strength of the theory of, 148
Grimaldi, his discovery with respect to light, 56
----Young's generalizations of, 56
Hamilton, Sir William, of Dublin, his discovery of conical refraction, 209
Heat, generation of, 6
----Dr. Draper's investigation respecting, 171
Helmholtz, his estimate of the genius of Young, 50
----on the imperfect achromatism of the eye, 29 _note_, 31
----reveals the cause of green in the case of pigments, 37
Henry, Professor Joseph, his invitation, 2
Herschel, Sir John, his theoretical calculations respecting
diffraction, 87
----first notices and describes the fluorescence of sulphate of quinine,
165
----his experiments on spectra, 201
Herschel, Sir William, his experiments on the heat of the various
colours of the solar spectrum, 171
Hooke, Robert, on the colours of thin plates, 67
----his remarks on the idea that light and heat are modes of motion, 68
Horse-chestnut bark, fluorescence of, 165
Huggins, Dr., his labours, 205
Huyghens advocates the conception of ether, 48, 58
----his celebrated principle, 83
Huyghens on the double refraction of Iceland spar, 112
Iceland spar, 109
----double refraction caused by, 110
----this double refraction first treated by Erasmus Bartholinus, 112
----character of the beams emergent from, 114
----tested by tourmaline, 116
----Knoblauch's demonstration of the double refraction of, 185
Ice-lens, combustion through, 167
Imagination, scope of the, 42
----note by Maclaurin on this point, 43 _note_
Janssen, M., on the rose-coloured solar prominences, 204
Jupiter, Roemer's observations of the moons of, 20
Jupiter's distance from the sun, 20
Kepler, his investigations on the refraction of light, 14, 207
Kirchhoff, Professor, his explanation of Fraunhofer's lines, 193
----his precursors, 201
----his claims, 203
Knoblauch, his demonstration of the double refraction of heat of
Iceland spar, 185
Lactantius, on the natural philosophers of his time, 13
Lamy, M., isolates thallium in ingots, 193
Lesley, Professor, his invitation, 2
Light familiar to the ancients, 5
----generation of, 6, 7
----spherical aberration of, 8
----the rectilineal propagation of, and mode of producing it, 9
----illustration showing that the angle of incidence is equal to the
angle of reflection, 10, 11
----sterility of the Middle Ages, 13
----history of refraction, 14
----demonstration of the fact of refraction, 14
----partial and total reflection of, 16-20
----velocity of, 20
----Bradley's discovery of the aberration of light, 21, 22
----principle of least time, 23
----Descartes and the rainbow, 24
----Newton's analysis of, 26, 27
----synthesis of white light, 30
----complementary colours, 31
----yellow and blue lights produce white by their mixture, 31
----what is the meaning of blackness? 32
----analysis of the action of pigments upon, 33
----absorption, 34
----mixture of pigments contrasted with mixture of lights, 37
----Wünsch on three simple colours in white light, 39 _note_
----Newton arrives at the emission theory, 45
----Young's discovery of the undulatory theory, 49
----illustrations of wave-motion, 58
----interference of sound-waves, 58
----velocity of, 60
----principle of interference of waves of, 61
----phenomena which first suggested the undulatory theory 62-69
----soap-bubbles and their colours, 62-65
----Newton's rings, 69-77
----his espousal of the emission theory, and the results of this
espousal, 77
----transmitted light, 77
----diffraction, 77, 89
----origin of the notion of the attraction of gravitation, 92
----polarity, how generated, 93
----action of crystals upon, 98
----refraction of, 106
----elasticity and density, 108
----double refraction, 109
----chromatic phenomena produced by crystals in polarized, 121
----the Nicol prism, 122
----mechanism of, 125
----vibrations, 125
----composition and resolution of vibrations, 128
----polarizer and analyzer, 127
----recompounding the two systems of waves by the analyzer, 129
----interference thus rendered possible, 131
----chromatic phenomena produced by quartz, 139
----magnetization, of, 141
----rings surrounding the axes of crystals, 143
----colour and polarization of sky, 149, 154
----range of vision incommensurate with range of radiation, 159
----effect of thallene on the spectrum, 162
----fluorescence, 162
----transparency, 167
----the ultra-red rays, 170
----part played in Nature by these rays, 175
----conversion of heat-rays into light-rays, 176
----identity of radiant heat and, 177
----polarization of heat, 180
----principles of spectrum analysis, 189
----spectra of incandescent vapours, 190
----Fraunhofer's lines, and Kirchhoff's explanation of them, 193
----solar chemistry, 195-197
----demonstration of analogy between sound and, 198, 199
----Kirchhoff and his precursors, 201
----rose-coloured solar prominences, 204
----results obtained by various workers, 205
----summary and conclusion, 206
----polarized, the spectra of, 227
----measurement of the waves of, 234
Lignum Nephriticum, fluorescence of, 164
Lloyd, Dr., on polarization of heat, 180, 209
Lockyer, Mr., on the rose-coloured solar prominences, 205
Lycopodium, diffraction effects caused by the spores of, 88
Magnetization of light, 141
Malus, his discovery respecting reflected light through Iceland spar, 115
----discovers the polarization of light by reflection, 208
Masson, his essay on the bands of the induction spark, 202
Melloni, on the polarization of heat, 180
Metals, combustion of, 5, 6
----spectrum analysis of, 190
----spectrum bands proved by Bunsen and Kirchhoff to be characteristic
of the vapour of, 192
Mill, John Stuart, his scepticism regarding the undulatory theory, 149
Miller, Dr., his drawings and descriptions of the spectra of various
coloured flames, 201
Morton, Professor, his discovery of thallene, 162
Mother-of-pearl, colours of, 90
Nature, a savage's interpretation of, 4
Newton, Sir Isaac, his experiments on the composition of solar light, 26
----his spectrum, 27
----dispersion, 27
----arrives at the emission theory of light, 45
----his objection to the conception of an ether espoused and defended by
Huyghens and Euler, 58
----his optical career, 70
----his rings, 69-77
----his rings explained by the theory of 'fits,' 73
----espouses the emission theory, 77
----effects of this espousal, 77
----his idea of gravitation, 92
----his errors, 208
Nicol prism, the, 122
Ocean, colour of the, 35
OErsted, discovers the deflection of a magnetic needle by an electric
current, 176
Optics, science of, 4
Pasteur referred to, 219
Physical theories, origin of, 41-44
Pigments, analysis of the action of, upon light, 33
----mixture of, contrasted with mixture of lights, 37
----Helmholtz reveals the cause of the green in the case of mixed blue
and yellow pigments, 37
----impurity of natural colours, 37
Pitch of sound, 59
Plücker, his drawings of spectra, 202
Polariscope, stained glass in the, 130,131
----unannealed glass in the, 136
Polarity, notion of, how generated, 93
----atomic, 93-96
----structural arrangements due to, 96
----polarization of light, 112
----tested by tourmaline, 116
----and by reflection and refraction, 119
----depolarization, 120
Polarization of light, 112
----circular, 140
----sky-light, 149, 157
----of artificial sky, 156
----of radiant heat, 180
Polarizer and analyzer, 127
Poles of a magnet, 93
Powell, Professor, on polarization of heat, 180
Prism, the Nicol, 122
Quartz, chromatic phenomena produced by, 139
Radiant heat, 172
----diathermancy, or perviousness to radiant heat, 173
----conversion of heat-rays into light rays, 174
----formation of invisible heat-images, 179
----polarization of, 180
----double refraction, 182
----magnetization of, 184
Rainbow, Descartes' explanation of the, 24
Refraction, demonstration of, 14
Refraction of light, 106
----double, 109
Reflection, partial and total, 16-20
Respighi, results obtained by, 205
Ritter, his discovery of the ultraviolet rays of the sun, 159
Roemer, Olav, his observations of Jupiter's moons, 20
----his determination of the velocity of light, 21
Rubidium, discovery of, 193
Rusting of iron, what it is, 5
Schwerd, his observations respecting diffraction, 87
Science, growth of, 176, 203
Scoresby, Dr., succeeds in exploding gunpowder by the sun's rays
conveyed by large lenses of ice, 167
Secchi, results obtained by, 205
Seebeck, Thomas, discovers thermo-electricity, 176
----discovers the polarization of light by tourmaline, 208
Selenite, experiments with thick and thin plates of, 124
Silver spectrum, analysis of, 190, 191
Sky-light, colour and polarization of, 149, 154
----generation of artificial skies, 152
Snell, Willebrord, his discovery, 14
----his law, 15, 24
Soap-bubbles and their colours, 63, 65
Sound, early notions of the ancients respecting, 51
----interference of waves of, 58
----pitch of, 59
----analogies of light and, 56
----demonstration of analogy between, and light, 198, 199
Sonorous vibrations, action of, 134
Spectrum analysis, principles of, 189
Spectra of incandescent vapours, 190
----discontinuous, 191, 192
----of polarized light, 227
Spectrum bands proved by Bunsen and Kirchhoff to be characteristic of
the vapour, 192
----its capacity as an agent of discovery, 193
----analysis of the sun and stars, 193
Spottiswoode, Mr. William, 123, 227
Stewart, Professor Balfour, 202
Stokes, Professor, results of his examination of substances excited by
the ultra-violet waves, 161
----his discovery of fluorescence, 162
----on fluorescence, 165
----nearly anticipates Kirchhoff's discovery, 198, 202
Striated surfaces, colours of, 89
Sulphate of quinine first noticed and described by Sir John Herschel, 165
Sun, chemistry of the, 195
Sun, rose-coloured solar prominences, 204
Talbot, Mr., his experiments, 201
Tartaric acid, irregular crystallization of, and its effects, 131
Thallene, its effect on the spectrum, 162
Thallium, spectrum analysis of, 190, 191
----discovery of, 193
----isolated in ingots by M. Lamy, 193
Theory, relation of, to experience, 91
Thermo-electric pile, 176
Thermo-electricity, discovery of, 176
Tombeline, Mont, inverted image of, 19
Tourmaline, polarization of light by means of, 112
Transmitted light, reason for, 77
Transparency, remarks on, 167
Ultra-violet sun-rays, discovered by Ritter, 159
----effects of, 160
Ultra-red rays of the solar spectrum, 171
----part played by the, 173
Undulatory theory of light, bases of the, 47
----Sir David Brewster's chief objection to the, 47
Undulatory theory of light, Young's foundation of the, 49
----phenomena which first suggested the, 62, 69
----Mr. Mill's scepticism regarding the, 143
----a demonstrated verity in the hands of Young, 210
Vassenius describes the rose-coloured solar prominences in 1733, 204
Vitellio, his skill and conscientiousness, 14
----his investigations respecting light, 207
Voltaic battery, use of, and its production of heat, 6, 7
Water, deportment of, considered and explained, 105, 106
Waves of water, 51
----length of a wave, 52
----interference of waves, 53-55
Wertheim, M., his instrument for the determination of strains and
pressures by the colours of polarized light, 134
Wheatstone, Sir Charles, his analysis of the light of the electric
spark, 202
Whirlpool Rapids, illustration of the principle of the interference of
waves at the, 55
Willigen, Van der, his drawings of spectra, 202
Wollaston, Dr., first observes lines in solar spectrum, 193
----discovers the rings of Iceland spar, 209
Woodbury, Mr., on the impurity of natural colours, 37
Wünsch, Christian Ernst, on the three simple colours in white
lights, 39 _note_
----his experiments, 39 _note_
Young, Dr. Thomas, his discovery of Egyptian hieroglyphics, 49;
----and the undulatory theory of light, 49
----Helmholtz's estimate of him, 50
----ridiculed by Brougham in the 'Edinburgh Review,' 50
----generalizes Grimaldi's observation on light, 56, 57
----photographs the ultra-violet rings of Newton, 160