_Camera lucida._ A lens is necessary to reduce to the same apparent distance the two objects seen simultaneously. Instruments to assist the sight; simple microscope; the magnifying power; distinctness; field; advantage of a diaphragm; it modifies the field and the brightness variously according to its position. Woollaston’s double glass; its advantages. General principle of compound dioptrical instruments. Compound microscope; experimental measure of its magnifying power, by means of the diaphragm, by means of the camera lucida. Astronomical telescope; object glass; simple eye-glass. Necessity for a diaphragm; its place; the wires, their place; optic axis of a telescope. Parallax of the threads of the wires; magnifying power of the object-glass; of the eye-glass; field of view of a telescope. Optic ring; different methods of measuring the magnifying power. Distinctness of a telescope; night-glass. Different distances of drawing out the eye-glass for short-sighted and long-sighted observers. Different sorts of eye-pieces; positive eye-pieces; ordinary double eye-piece of the astronomical telescope. Ramsden’s eye-piece; treble eye-piece of the terrestrial telescope. Negative eye-pieces; simple eye-piece of Galileo. Compound _ditto_ of Huyghens; advantages and disadvantages of these different combinations; general principle of catadioptrical instruments. LESSONS 27-29. _Double Refraction._ Crystallized mediums do not all act upon light like homogeneous mediums. Double refraction of Iceland spar: the extraordinary image turns round the ordinary image. The ordinary and extraordinary rays cross at the interior of the crystal. Huyghens’ construction; measure of the ordinary and extraordinary indices of refraction; attractive and repulsive crystals; a ray falling perpendicularly does not always bifurcate in a camera with parallel faces, nor in a prism. Definition of uniaxial and biaxial crystals. The dispersion of the ordinary ray differs from that of the extraordinary ray. The two rays are unequally absorbed in many colored mediums. Tourmaline. Doubly-refracting prisms; their construction. Use of doubly-refracting prisms to measure apparent diameters, &c. LESSONS 30-31. _Polarization._ Successive refractions in doubly-refracting prisms. Special properties of the two rays emerging from the first doubly refracting crystal. Polarization by double refraction. Reflection from transparent media polarizes the light partially or wholly according to the incidence. Brewster’s law. Reflection of polarized light from a transparent medium. Simple refraction partially polarizes the light. Many successive refractions polarize it almost totally. Piles of glasses. Different methods to obtain a ray of polarized light, 1st, by reflection; 2nd, by simple refraction; 3rd, by double refraction, by eliminating one of the refracted pencils;--by a screen,--by total reflection, Nicol’s prism, by absoption, tourmaline.