Iceland spar) cut perpendicularly to its axis, and the emergent light be afterwards analyzed, it presents a system of rings and cross different to those obtained from either plane or circularly polarized light.

The preceding are some only of the peculiarities which distinguish this from other kinds of light; and in conclusion, I may observe, that elliptical polarization forms a connecting link between plane and circular polarization.

5. MACLES AND COMPOUND CRYSTALS.

I have now arrived at the last part of my subject, viz., the consideration of the optical properties of those remarkable crystalline structures commonly known by the name of macles, a term introduced into mineralogy by Romé de Lisle. Sometimes these structures appear to consist of one crystal, whose parts are transposed, dislocated, or displaced. When one-half of the crystal appears to have been turned partly round on an imaginary axis, passing through the centre of the crystal, and perpendicularly to the plane of section, and to have been united to the other half in this position, the body thus produced has been called the hemitrope (from μ half and трéпw I turn). Of this arrowheaded selenite is a familiar example. Sometimes two or more crystals are found intersecting each other, and are then called intersecting crystals. When two crystals are joined, they form the structure called a twin or double crystal.

Many or most of the forms I have now referred to are irregular, and might appear to be accidental. But there are some others which have great regularity, and cannot be ascribed to accident. Such are some specimens of apophyllite and sulphate of potash. They constitute what Dr. Brewster has termed tesselated or composite crystals; the real structure of many of which is only discoverable by the aid of polarized light; they consist of several crystals, or portions of crystals, juxtaposed, or united so as to form a compound crystal, the figure of which is very different from that of the crystals composing it.

Macled crystals of nitre and arragonite are very common, and frequently their precise structure is undiscoverable by the naked

eye.

In quartz we often find right and left-handed crystals intersecting each other.

Amethyst (by many mineralogists considered to be a variety of quartz), is a remarkable example of a combination of right and left-handed varieties of quartz. If a plate of amethyst, cut perpendicularly to the principal axis of the crystal, be examined by the polariscope, it presents a striped or fringed appearance,

variegated with the most gorgeous and brilliant tints. This is owing to its being composed of alternate minute strata of right and left-handed quartz, whose planes of polarization are parallel to the principal axis of the prism.

Topaz sometimes presents a remarkably composite structure. It belongs to the right rhombic system, and presents, when regularly formed, two systems of rings. Cut at right angles to the axis it often presents a central rhomb, "surrounded by a border in which the optic meridians of the alternate sides are inclined at a quarter of a right angle to that of the central compartment, and half of a right angle to each other. In consequence, when such a rhombic plate is held with its long diagonal in the plane of primitive polarization, two opposite sides of the border appear bright, the other two black, and the central compartment of intermediate brightness. Such specimens often present the phenomena of dichroism in the central compartment, while the border is colourless in all positions"."

Sulphate of potash is composed of six crystals belonging to the right prismatic system, joined so as to form a single or double six-sided pyramid, and simulating the crystals of the rhombohedric system. When, therefore, we put a slice of it, cut at right angles, to the axis of the pyramid, in the polariscope, we observe not a circular cross and rings, but a tesselated structure.

One variety of apophyllite, called tesselite, presents a remarkable structure of the same kind; but its phenomena are still more extraordinary. Apophyllite is composed principally of silicate of lime, with a little silicate of potash. It crystallizes in right square prisms. Plates cut transversely to the axis, and examined by polarized light, appear to consist of nine crystals contained within a number of parallel veins or plates. The central crystal has only one axis of no double refraction, the others two. (See p. 68).

Analcime or Cubizite is another remarkable crystal. It consists principally of silicate of alumina with silicate of soda. It crystallizes in the form of the cube, or some form allied to this, as the icosatetrahedron. Instead of being without double refraction, as cubical crystals usually are, it presents a number of planes of no double refraction. It is, therefore, a compound crystal, that is, is composed of a number of crystalline parts disposed symmetrically. (See p. 66).

And here also may be noticed what has been called interrupted Iceland spar. Some specimens of Iceland spar give four or even more images, which sometimes exhibit complementary tints. They owe this property to the presence of one or more intersecting

* Herschel, Encycl. Metrop.

or interrupting films or strata of the same substance, placed perpendicularly to the short diagonal of the faces of the crystal. This film acts like the depolarizing plate in the polariscope, while the two portions of the crystal between which it is placed, act, the one as the polarizer, the other as the analyzer of the polariscope. Crystals like these, which thus exhibit their colours and rings per se, that is without the polariscope, have been called by Sir John Herschel, idiocyclophanous (from idios proper, kúkλOS a circle, and paiva I appear). Similar phenomena are sometimes exhibited by crystals of nitre, and still more frequently by bicarbonate of potash.

Conclusion.-The lateness of the hour compels me to avoid any lengthened peroration. I shall, therefore, content myself with thanking my auditors for the great attention which they have manifested during the entire course.

THE END.

C. WHITING, BEAUFORT HOUSE, STRAND.