Page:Popular Science Monthly Volume 6.djvu/403

Rh of them, or over a sufficient number, being noted. The method of noting this time may be best understood by referring to Fig. 2.

Suppose that the line in the middle of the figure is one of the transit-threads, and that the star is passing from the right hand of the figure toward the left: if it is on this wire at an exact second by the clock (which is always near the observer, beating seconds audibly), this second must be written down as the time of the transit over this thread. As a rule, however, the transit cannot occur on the exact beat of the clock, but at the seventeenth second (for example) the star will be on the right of the wire, say at a; while, at the eighteenth second, it will have passed this wire and may be at b. If the distance of a from the wire is six-tenths of the distance a b, then the time of transit is to be recorded as—hours—minutes (to be taken from the clock-face), and seventeen and six-tenths seconds; and in this way the transit over each wire is observed. This is the method of "eye-and-ear" observation, the basis of such work as we have described, and it is so called from the part which both the eye and the ear play in the appreciation of intervals of time. The ear catches the beat of the clock, the eye fixes the place of the star at a; at the next beat of the clock the eye fixes the star at b, and subdivides the space a b into tenths, at the same time appreciating the ratio which the distance from the



thread to a bears to the distance a b. This is recorded as above. Now, if the action of the eye and the ear and the coördinating action of the brain (which must associate some spot in the field of view with some second) were all instantaneous in their action, the phenomenon of personal equation would not exist. As a matter of fact, when the clock beats and the star is really at a, the mind refers it to some point farther on in the field as a'; and when the clock again beats, the star, which truly is at b, is by the mind referred to a point b'. The distance a b is the same as a' b'; but the distance from the thread to a is greater than the distance from the thread to a'. Hence, instead of recording the time of transit as 17s.6, an observer, whose habit is correctly represented by the figure, might record this time as 17s.4, and the correction $$+$$ 0s.2 would be required to be applied to his times of transit to reduce them to the exact truth: $$+$$ 0s.2 is then his absolute personal correction. But, in general, we have no means of determining where a and b, in our field of view, are, and hence the knowledge of the absolute personal equation has to be gained by some special