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.