Pike's Illustrated Descriptive Catalogue of Optical, Mathematical, and Philosophical Instruments, Manufactured, Imported, and Sold by the Author: With the Prices Affixed at which They are Offered in 1856. With Upwards of 750 Engravings ...

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The author, 1848
 

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Page 79 - ... well as any other; for the rays incident on the index-glass will pass through the transparent half of the horizonglass, without much diminution of their brightness. " The advantages of this instrument, when compared with the sextant, are chiefly these : the observations for finding the indexerror are rendered useless, all knowledge of that being put out of the question, by observing both forwards and backwards. By the same means the errors of the...
Page 326 - A voltaic series fixed in a trough is combined with another trough destitute of plates, and of a capacity sufficient to hold all the acid necessary for an ample charge. The trough containing the series is joined to the other lengthwise, edge to edge ; so that, when the sides of the one are vertical, those of the other must be horizontal. The advantage of this...
Page 129 - ... half the error, in the same manner, must be gone through again, until, by successive approximations, the object is found to be bisected in both positions of the axis ; the adjustment will then be perfect. The collimation adjustment may likewise be examined from time to time, by observing the transit of Polaris, or any other close circumpolar star, over the first three wires, which gives the intervals in time from the first to the second, and from the first to the third wire ; and then reversing...
Page 85 - The Adjustments. The first adjustment is that of the line of collimation, — that is, to make the intersection of the cross wires coincide with the axis of the cylindrical rings on which the telescope turns : it is known to be correct, when an eye looking through the telescope observes their intersection continue on the same point of a distant object during an entire revolution of the telescope. The usual method of making this adjustment is as follows...
Page 57 - The perpendicular thread of the sight-vane, E, and the divisions on the card, appear together on looking through the prism, and the division with which the thread coincides, when the needle is at rest, is the
Page 337 - A piece fig. 399. of platinized silver has a bar of wood fixed on the top to prevent contact with the zinc, and is furnished with a binding screw. A stout plate of well amalgamated zinc, of the width of the silver, is placed on each side of the wood, and both are held in their place by a binding screw sufficiently wide to embrace the zincs and the wood. This arrangement is immersed in a jar or glass, containing dilute sulphuric acid, in the proportion of one of acid to seven of water, and not the...
Page 77 - Prepare the instrument for observation by screwing the telescope into its place, adjusting the drawer to focus, and the wires parallel to the plane, exactly as you do with a sextant : also set the index forwards to the rough distance of the sun and moon, or moon and star; and, holding the circle by the short handle, direct the telescope to the fainter object, and make the contact in the usual way. Now read off the degree, minute, and second, by that branch of the index to which the...
Page 86 - ... any deviation in it is easily rectified, by releasing the screws by which it is held, and tightening them again after having made the adjustment ; or, what is perhaps better, note the quantity of deviation as an index error, and apply it, plus or minus, to each vertical angle observed. This deviation is best determined by repeating the observation of an altitude or depression in the reversed positions, both of the telescope and the vernier plate: the two readings will have equal and opposite...
Page 290 - VIII) is from one and a half to two and a half inches in length, and from one-fourth to three fourths of an inch in diameter.
Page 96 - ... corresponds with the horizontal wire of the telescope, the height of the wire in the vane, noted on the staff, is the height of the apparent level above the ground at that place. When both the staves are used, they should be set up at equal distances on each side of the spiritlevel : the difference of the heights of their vanes will be the absolute difference of level between the two stations. But when one staff only is employed, the difference between the height of the vane and the height of...

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