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Ice is a familiar example: its molecules are built with perfect symmetry around the perpendiculars to the planes of freezing, and a ray sent through ice in this direction is not doubly refracted; whereas, in all other directions, it is. Iceland spar is another example of the same kind: its molecules are built symmetrically round the line uniting the two blunt angles of the rhomb.
A. The sides of the rhomboid are not equal, nor yet its angles, but the sides of the rhomb are equal. Trapezoid. Q. What is this. A. A trapezoid. Q. How many sides has it? A. Four sides and four angles, it has only two of its angles equal, which are opposite to each other. Tetragon. Q. What do we call these figures that have four sides. A. Tetragons, tetra meaning four.
Q. How many sides and angles has it? A. Four, the same as a square. Q. What is the difference between a rectangle and a square? A. A rectangle has two long sides, and the other two are much shorter, but a square has its sides equal. Rhomb. Q. What is this? A. A rhomb. Q. What is the difference between a rhomb and a rectangle?
The requisite obliquity is secured by making the rhomb of such a length that the plane of which b c is the section shall be perpendicular, or nearly so, to the two end surfaces of the rhomb b x, c y.
The four-sided pyramid, perhaps the most frequent of all natural crystals, is called in architecture a dogtooth; its use is quite limitless, and always beautiful: the cube and rhomb are almost equally frequent in chequers and dentils: and all mouldings of the middle Gothic are little more than representations of the canaliculated crystals of the beryl, and such other minerals: § XXIV. Not knowingly.
No. 9 monitor has another set of geometrical definitions on the same principle, as a perpendicular line, a horizontal line, an oblique line, parallel lines, curved lines, diverging or converging lines, an obtuse angle, a circle. No. 10 a different set of geometrical shapes, viz. sociles-triangles, scolene-triangles, rectangle, rhomb, rhomboid, trapezoid, trapeziums, ellipse or oval.
Thus, by a drawbridge spanning dark, oily waters, they came into a vast courtyard and an atmosphere as of mildew. A studded door stood ajar, and through the gap, from a guiding beacon of infamy, fell a rhomb of yellow light, suddenly obscured by a squat female figure when the steps of the Marchioness and her companions fell upon the stones of the yard.
The question must therefore lie between the two others, and it is as yet impossible to decide which of them is best entitled to the epithet, as their sources lie very nearly in the same northeast and southwest rhomb line. Another circumstance would, however, render the decision between them easy.
When, for example, in the demonstration of the theorem of Pythagoras, children handle the various pieces of the metal insets, they should start from the point at which they become aware that a rectangle is equal to the rhomb, and a square is equal to the same rhomb.
Let this rhomb be cut along the plane b c; and the two severed surfaces, after having been polished, reunited by Canada balsam. We learned, in our first lecture, that total reflection only takes place when a ray seeks to escape from a more refracting to a less refracting medium, and that it always, under these circumstances, takes place when the obliquity is sufficient.
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