The Monster Telescopes, Erected by the Earl of Rosse, Parsonstown

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: surface producing a spherical figure. According, however, as the focal length (the actual average amount of abrasion during a given time being given) increases more or less rapidly, the nature of the curve will vary, and we might conceive it possible, having it in our power completely to control the rate...
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Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: surface producing a spherical figure. According, however, as the focal length (the actual average amount of abrasion during a given time being given) increases more or less rapidly, the nature of the curve will vary, and we might conceive it possible, having it in our power completely to control the rate at which the focal length increases, so to proportion the rate of that increase, as to produce a surface approximating to that of the paraboloid. Of course the chances against obtaining an exact paraboloid are infinitely great, as an infinite number of curves may pass between the parabola and its circle of curvature, and it is vain to look for a guide in searching for the proper one in calculations founded on the principles of exact science, as the effect of friction in polishing is not conformable to any known law; still from a number of experiments it might be possible to deduce an empirical formula practically valuable: this I have endeavoured to accomplish. The grinding of the Speculum to the proper figure then depends on the relative velocities of the different parts, as before stated. The substance made use of to wear down the surface was emery and water; a constant supply of these was kept between the grinder and the Speculum. The Grinder is made of cast iron, with grooves cut lengthways, across, and circularly, on its] face; there are twenty- fiye grooves, crossed by as many more, which are quarter of an inch wide, and half an inch deep The circular grooves, of which there are thirteen concentric with the polisher, are three-eighths of an inch deep, and quarter of an inch wide. The polisher and speculum have a mutual action on each other; in a few hours, by the help of the emery and water, they are both ground truly circular, whatever might have been their previ...