[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 reconnaitre que parmi les peuples civilises de nos jours il en est pen chez qui les hautes sciences aient fait moins de progres qu’aux Etats-Unis, ou qui aient fourni moins de grands artistes, de poetes illustres et de celebres ecrivains.’ (De la Democratie en Amerique, 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

Angstroem, his paper on spectrum analysis, 202

Arago, Francois, 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

Berard 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

Caesium, 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