Star-land : being talks with young people about the wonders of the heavens (1891)
Ball, Robert S. (Robert Stawell), Sir, 1840-1913 · London ; Paris : Cassell & Co. · leaf 269
t is essential that themirror shall do this work accurately, and this it will onlydo when it has been truly shaped so as to be a parabola. 254 Star-Land. You will remember, also, how I described to you the re-flector which Herschel made for his great telescope (Fig. 67).The shape of the mirror must… … Fig. 73—The Lighthouse Reflector. comets pursue. Unlike the ellipse, the parabola is an opencurve ■ it has two branches stretching away and away forever, and always getting further apart. Of course, in theexamples of this curve that I have given, it is only a smallpart of the figure that is…
Astronomy
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