A treatise on the principal mathematical instruments employed in surveying, levelling, and astronomy: explaining their construction, adjustments, and use. With an appendix, and tables...

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Troughton and Simms, 1836 - Astronomical instruments - 118 pages
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Page 48 - ... to make the objects appear on the other wire ; if the contact still remains perfect, the axis of the telescope is in proper adjustment ; if not, it must be altered by moving the two screws which fasten, to the up-and-down piece, the collar into which the telescope screws. This adjustment is not very liable to be deranged.
Page 23 - 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...
Page 64 - ... 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 the axis, observe the same intervals in a reverse order, as the wires which were the three first, in the former...
Page 54 - But what is still of more consequence, the error of the centre is perfectly corrected, by reading the three branches of the index ; while this property combined with that of observing both ways, probably reduces the errors of dividing to one-sixth part of their simple value. Moreover, angles may be measured as far as one hundred and fifty degrees, consequently the sun's double altitude may be observed when his distance from the zenith is not less than fifteen degrees ; at which altitude, the head...
Page 17 - ... 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 47 - The amount of the index error may be found in the following manner: clamp the index at about 30 minutes to the left of zero, and looking towards the sun, the two images will appear either nearly in contact or overlapping each other ; then perfect the contact, by moving the tangent-screw, and call the minutes and seconds denoted by the vernier, the reading on the arc. Next place the index about the same quantity to the right of zero, or on the arc of excess, and make the contact of the two images...
Page 31 - Now, if the stake 6 be half way between a and c,f then ought c" — c' — (A" — A) to be equal to 2 [B" — B'— (A"— A')] ; but if not, alter the screws which adjust the diaphragm, and consequently the horizontal spider line, or wire, until such be the case ; and then the instrument will be adjusted for collimation. " To adjust the spirit-bubble without removing the' instrument, read the staff A, say it reads A'", then adding (A'"— A') with its proper sign to B
Page 35 - AC 2AC nearly ; that is, the difference between the true and apparent level is equal to the square of the distance between the places, divided by the diameter of the earth ; and consequently it is always proportional to the square of the distance.
Page 24 - Place the telescope over two of the parallel plate screws, and move them, unscrewing one while screwing up the other, until the bubble of the level settles in the centre of its run ; then turn the instrument half round upon the vertical axis, so that the contrary ends of the telescope may be over the same two screws, and, if the bubble does not again settle at the same point as before, half the error must be corrected by turning...
Page 63 - ... h. The whole of this apparatus is movable with the telescope, and when the axis is reversed, can be attached in the same manner to the opposite standard. Near the eye -end, and in the principal focus of the telescope, is placed the diaphragm, or wire-plate, which, in the theodolite or levelling telescope, need only carry two cross wires, but in this instrument it has five vertical and two horizontal wires. The centre vertical wire ought to be fixed in the optical axis of the telescope, and perpendicular...

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