Stargazing, Past and Present. Expanded from Shorthand Notes of a Course of Lects., with the Assistance of G.M. Seabroke (Paperback)

This historic book may have numerous typos and missing text. Purchasers can download a free scanned copy of the original book (without typos) from the publisher. Not indexed. Not illustrated. 1878 Excerpt: ...Double Image Micrometer. Fio. a, p -I, single linage formed by objectglmw. Fio. I, pf., p.2 72, images separated by the double refracting prism. Fio. e, same, separate' less, by the motion of the prism. object-glass. Mr. Browning has constructed micrometers on this principle. There is yet another double-image micrometer depending on the power of a prism to alter the direction of rays of light. It is constructed by making two very weak prisms, i.e.. having their sides very nearly parallel, and cutting them to a circular shape; these are mounted in a frame one over the other with power to turn one round, so that in one position they both act in the same direction, and in the opposite one they neutralise each other; these are carried by radial arms, and are placed either in front of the object-glass or at such a distance from it inside the telescope that they intercept one half of the light, and the remaining portion goes to form the usual image, while the other is altered in its course by the prism and forms another image, and this alteration depends on the position of the movable prism. BOOK IV. MODERN MERIDIONAL OBSERVATIONS. CHAPTER XVr. THE TRANSIT CIRCLE. We are now, then, in full possession of the stock-intrade of the modern astronomer--the telescope, the clock, and the circle, --and we have first to deal with what is termed astronomy of position, that branch of the subject which enables us to determine the exact position of the heavenly bodies in the celestial sphere at any instant of time. Before, however, we proceed with modern methods, it will be well, on the principle of reculer pour mieux sauter, to refer back to the old ones in order that we can the better see how the modern instruments are arranged for doing the work which Tycho, for instan...