A Treatise on New Philosophical Instruments for Various Purposes in the Arts and Sciences (Electronic book text)

Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. Excerpt from book: Section 3CHAP. II. textit{Description of a New Micrometer for Reflecting textit{Telescopes. In applying to the reflecting telescopes of Gregory and Cassegrain the principle which has been explained in the preceding Chapter, we are led to the formation of a micrometer, remarkable for the simplicity of its construction; and what, at first sight, may appear paradoxical, we may convert a Gregorian or a Cassegrainian telescope into a very accurate micrometer, almost without the aid of any additional apparatus. It will be readily seen by those who understand the theory of these telescopes, that their magnifying power may be increased merely by varying the distance between the eyepiece and the great speculum; and then producing distinct vision by anew adjustment of the small mirror. Hence a pair of wires fixed in the eyepiece may be made to subtend different angles, solely by having thatpart of the instrument moveable along a portion of the common axis of the two mirrors. In order to understand this, let SS, Plate II. Fig. 3. be the great speculum of a Gregorian telescope, having a round hole in its centre, and placed at the extremity of the tube AA; and let M be the small speculum, whose focus is G, and centre H, attached to an arm MQ, and moveable along the axis of the instrument by means of a screw and milled head. The rays RR, proceeding from the lower part of any object, and falling upon the speculum SS, will be reflected to R', and will there form an image of that part of the object. In like manner, the rays textit{rr will form an image of the upper part of the object at textit{r'. The rays diverging from the image R'r', and intercepted by the small speculum M, will form another image R"r," at the distance MF; which being viewed by the eyeglass at E, whose focal distance is FE, will appear distinct a...