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