Theoretical ideas upon the propagation of a vibratory motion in indefinite elastic media, on an indefinite cylindrical tube. Waves of condensation of dilatation. Progressive nodes and ventral divisions. Laws of the intensities of sound. Direct measure of the velocity of the propagation of sound in water. Measure of the velocity of the propagation of sound in air. Formulæ without demonstration. Comparison of the formulæ with experiment. Sonorous waves reflected in an indefinite medium. Fixed nodes and ventral divisions. Sonorous waves reflected in closed and open tubes. Fixed nodes and ventral divisions; the vibratory state and density thereat. Series of sounds afforded by the same tube. Effect of holes. Sonorous reflected waves in rods. Series of sounds afforded by the same rod vibrating longitudinally. Indirect measure of the velocity of sound in gases, liquids, and solids. Experiments on the communication of vibrating motion in heterogeneous mediums, on the general direction of the vibrating motion communicated. Intensification of sounds. Interferences. Beats. Different stringed and wind instruments. Means of setting them in vibration. A few words on the organs of voice and hearing. Incompleteness of our knowledge on this subject. OPTICS. LESSONS 16-17. _Propagation of Light._ Propagation of light in a straight line. Rays of light. Geometrical theory of shadows. Velocity of light. Rœmer’s observations. Laws of intensity of light. Photometers of Bouguer, Rumford. Intensity of oblique rays. Comparison of illuminating powers. Total brightness. Intrinsic brightness. _Reflection._ Reflection of light: its laws. Experimental demonstration. Images formed by one or more plane mirrors. To ascertain if a looking-glass has its two faces parallel. Spherical mirrors. Foci, formulæ. Discussion. Images by reflection. Measure of the radius of a spherical mirror. Definition of caustics by reflection. Definition of the two spherical aberrations in mirrors. Woollaston’s goniometer. LESSON 18. _Refraction._ Refraction of light in homogeneous mediums. Descartes’ law. Experimental demonstration for solids and liquids. Inverse return of the rays. Successive refractions. Indices of transmission in terms of the principal indices. Consequences of Descartes’ law. Total reflection. Manner of observing it. Irregular refractions. Mirage. Refraction is always accompanied with the accessory phenomenon of dispersion. Geometrical consequences of the law of refraction. Focus of a plane surface. Focus of a medium bounded by two parallel plane surfaces; by two plane surfaces inclined in the form of a prism. Foci of a spherical surface; of a medium limited by two spherical surfaces. Lenses. Formula for lenses. Discussion. Varieties of lenses. Optic center. Images. Measure of the focal distance of lenses. Definition of caustics by refraction. Definition of the two spherical aberrations of a lens. LESSONS 19-20. _Dispersion._