Page:Scientific Memoirs, Vol. 2 (1841).djvu/64

52 of the observations, the telescope must be directed towards the mark, and this examination must be repeated from time to time; and if a deviation is indicated in the optical axis, it must be again brought back to its original vertical plane. If the precaution is taken to note two other divisions on the wall, one on either side of the mark, they will furnish the means of estimating the amount of the requisite correction. But it should be remembered that these divisions, though they may be made to correspond exactly with the divisions of the scale, will not have exactly the same value in seconds. If no such auxiliary marks have been made, the amount of the connection must be judged of by the eye, in parts of the divisions of the scale itself.

The observations are made at the vertical thread; the horizontal thread serving merely to indicate nearly the middle of the former. In order that it should make no difference whether the parts of the scale appear somewhat higher or lower in the field of view, the cross threads must have such a position, that a fixed object, seen on their crossing, remains accurately on the vertical thread, when the telescope is moved somewhat up and down. The mark also serves for this verification, which, however, need not be frequently repeated when the position is left unchanged.

The plumb-line suspended from the centre of the object-glass must be so near the scale that the image of both may appear with the same distinctness in the telescope, and that thus the division covered by the line may be precisely determined. The scale must be so placed that its zero must correspond with the plumb-line, or the division which does so correspond must be taken as an arbitrary zero. The verifying the undisturbed state of the scale should be repeated from time to time in the course of the