(1) A _straight line_ is, of course, a line which runs uniformly in the same direction, and which is regular and without curves. A straight line is the shortest distance between two points. (2) When we say that lines are _parallel_, we mean that they lie so that every part of each is equally spaced from the other. (3) A line is _horizontal_ when it is parallel with the level surface of the Earth under it. (4) A line is _perpendicular_ to the surface of the Earth when it is _plumb_, that is, in a line with the center of the Earth. (5) A _vertical line_ generally means a plumb line. (6) A _right angle_ is formed when a vertical, or a perpendicular, line meets a horizontal line. (7) A _circle_ is a curved line, all points of which are equally distant from its center. _Circular_ means round like a circle. (8) By _diameter_ is meant a straight line drawn from one side, or half of a circle, to the opposite side through its center. (9) The _radius_ of a circle is a straight line drawn from the center of a circle, or a ball, to its circumference. (10) A _ring_ is a disk or object having a circular hole cut in its center. (11) An _arc of a circle_ is a part of a circle. (12) A _quarter circle_ is, of course, the one-fourth part of a circle. (13) A _tangent_ is a line which touches a curve but does not cut it. (14) An _ellipse_ is an oval figure, drawn on a plane surface. (15) The _equator_ is a circle which divides the Earth or other ball into equal parts, and is 90 degrees from the north and south poles. (16) The _ecliptic_ is a circle round the Sky in whose plane lies the orbit of the Earth. [Illustration: FIG. 185.--GEOMETRICAL FIGURES.] APPENDIX C THE GREEK ALPHABET Many of the brighter stars have names, as Aldebaran, Capella, Sirius, etc., but astronomers now indicate the stars of a constellation by the letters of the Greek alphabet. The usual method is to call the brightest star of a constellation α, that is, Alpha, the next brightest β which is Beta, and so on, but sometimes the stars are lettered in order of position in the sky. The following is the Greek alphabet: α Alpha ν Nu β Beta ξ Xi (Zi) γ Gamma ο Omicron δ Delta π Pi ε Epsilon ρ Rho ζ Zeta σ Sigma η Eta τ Tau θ Theta υ Upsilon ι Iota φ Phi κ Kappa χ Chi λ Lambda ψ Psi μ Mu ω Omega APPENDIX D STAR TESTS FOR EYESIGHT There are a number of stars which are considered to be good tests for the seeing power of the eyes. The faint stars of the Pleiades are a fine test of this kind; but usually these tests are double stars and while one will be bright and easily seen its companion will be very faint. The test is to see the faint one, and if you can see it you may consider you have very good eyesight. Eyesight tests are given on the following pages: