A Treatise on the Principal Mathematical Instruments Employed in Surveying, Levelling, and Astronomy...

Troughton and Simms, 1850 - Astronomical instruments - 130 pages

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Page 26 - 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 58 - The foregoing instructions for taking distances apply equally for taking altitudes by the sea or artificial horizon, they being no more than distances taken in a vertical plane. Meridian altitudes cannot, however, be taken both backwards and forwards the same day, because there is not time ; all...
Page 52 - If it require correcting, the arc will appear broken where the reflected and direct parts of the limb meet. This, in a well-made instrument, is seldom the case, unless the sextant has been exposed to rough treatment. As the glass is in the first instance set right by the maker, and firmly fixed in its place, its position is not liable to alter ; therefore no direct means are supplied for its adjustment.
Page 60 - 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...
Page 19 - ... 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 errors,...
Page 26 - ... for determining the axis of the tube, or line of collimation. By turning the milled-headed screw, A, on the side of the telescope, the internal tube, a, will be thrust outwards, which, carrying the object-glass, is by this means adjusted to its focal distance, so as to show a distant object distinctly.
Page 53 - 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 35 - 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 18 - ... must be repeated carefully, until the adjustment is satisfactory. A similar proceeding will also put the vertical line correct, or, rather, the point of intersection, when there are two oblique lines instead of a vertical one. The second adjustment is that which puts the level attached to the telescope parallel to the rectified line of collimation. The clips...
Page 39 - 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.