Page:Popular Science Monthly Volume 6.djvu/409

Rh we have seen, to reduce the observations of A to what they would have been if B had made them, it is simply necessary to know how much later B is in the habit of observing than A, and to apply this as a constant correction to A's work.

This may be done in practice by A and B observing the same star in the same telescope; A over the first ten wires (see Fig. 1), and B over the second ten.

A knowledge of the distances of the various wires from the middle wire enables us to compare A's work with B's, and A—B is the relative personal equation.

There is, however, a strong objection to this process: if personal equation is any thing, it is the difference between established habits; and, if A observes over ten wires, and then hastily rises to allow B to take his place at the instrument, both A's habits and B's are broken in upon, and the resulting personal equation is likely to be affected by this fact. In general, the way adopted is to allow A to observe several stars leisurely, and from them to determine the error of the clock; B does the same, and from his observations also a clock-error is found; the difference of these clock-errors, reduced to the same epoch, gives the relative equation of A and B.

Now if, instead of A registering his own observations on the chronograph (for example), we could have the star register its own transit, then B's observation, compared with this, would give at once an absolute equation. We cannot use the real star for this purpose; but several attempts have been made to construct an apparatus which should register the transit of an artificial star, which star could, at the same time, be observed. The principle of all of these machines is, in general, the same, and we will merely give a brief account of one which is now under trial by the Coast Survey.

The artificial star is produced by lamp-light falling upon a small hole in a blackened plate; this plate is given a motion laterally, and the small point of light passes from one side to the other of a plate of ground glass, upon which lines are ruled to represent the spider-lines of the reticle. As the artificial star passes each wire, an electric signal is recorded on the chronograph, and the observer can also record his signal; and thus on the same chronograph-sheet many observations of absolute personal equation can be permanently recorded. Any velocity can be given to the star, so that it may pass through the field of view as slowly as the pole-star, or as rapidly as a star at the equator.