very 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,