VI CONTENTS. 



Page. 



principles of the wave-theory. (38) Principle of least time. (39) In- 

 tensity of reflected and refracted light. (40) Experimentum crucis 

 between the two theories. (41, 42) M. Foucault's experiment; 

 velocity of light in air, and in water, determined experimentally. 

 (43) No effect of the Earth's motion on the amount of refraction 

 Arago's experiment. (44) M. Fizeau's experiment, 20 



CHAPTER III. 



DISPERSION. 



(45-47) Composition of solar light; seven principal colours. (48) Colour 

 and refrangibility are permanent and original affections. (49) Mix- 

 ture in the spectrum ; modes of redncing it. (50) Newton's method 

 of determining the refractive indices of the several species of simple 

 light. (51) Intensity of light in the spectrum. (52-54) Fixed lines of 

 the_spectrum ; observations of Fraunhofer. (55, 56) Dispersion pro- 

 duced by a thin prism ; measure of dispersive powers. (57) Disper- 

 sive powers are different in different substances ; experiments of 

 Newton and Dollond. (58, 59) Condition of achromatism ; disper- 

 sive powers compared experimentally. (60, 61) Physical explanation 

 of dispersion ; difficulty in the wave-theory removed by M. Cauchy. 

 (62, 63) Relation between velocity of propagation and wave-length. 

 (64, 65) Refractive index deduced in terms of the wave-length in 



(66) Limiting index, ....,..;...- ... 37 



CHAPTER IV. * 



ABSORPTION AND EMISSION. 



(67) Absorption of light ; its dependence on the thickness of the body 

 traversed. (68) Dichroism explained. (69) Phenomena of absorp- 

 tion analyzed. (70) Effects of Sun's rays threefold. Luminous, 

 calorific, and chemical effects equally absorbed. (71) Absorptive 

 properties of bodies different. (72) Absorption of light by gases. 

 (73, 74) Emission of light dependent on temperature. Light emitted 

 by solids, liquids, and gases. (75) Modes of exhibiting the phenomena 

 of emission. Lights emitted by the alkaline metals. (76, 77) Peculia- 

 rity of the light of incandescent gases ; spectral analysis. Effects of 

 temperature. (78) Reversal of the spectra, of the gases. (79) Rela- 

 tion between the emissive and absorptive powers law of Kirch off ; a 

 consequence of the laws of vibratory propagation. (80) Phenomena 

 of solar spectrum explained. Solar chemistry, ........ 53 



