The practical astronomer : comprising illustrations of light and colours, practical descriptions of all kinds of telescopes, the use of the equatorial-transit, circular, and other astronomical instruments, a particular account of the Earl of Rosse's large telescopes, and other topics connected with astronomy (1845)
Dick, Thomas, 1774-1857 · London : Seeley, Burnside, and Seeley · leaf 91
66 THE PRACTICAL ASTRONOMER. from the object AB, (fig. 7.) to the point F, aresaid to converge towards that point. All convexglasses cause parallel rays, which fall upon them … c to converge in a greater or less degree; and theyrender converging rays still more convergent. IfAB, fig. 7. represent a convex lens, and HGIparallel rays falling upon it, they will be re-fracted and converge towards the point F, whichis called the focus, or burning point; because,when the suns…
Astronomy · Astronomical instruments · Telescopes
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