Now, taking analyzing lines aa^{1} and bb^{1} in figure 31 as horizontal, and letting the sweep be inclined as a small angle δ with the analyzing lines, the number of lines required to cross the sweep in the direction *ab* perpendicular to analyzing lines will be increased and hence the value in years between two analyzing lines will be decreased; hence

*a*to

*b*.

If the fringe is perpendicular to the analyzing lines, its period is the distance ab in years and we have for this special case:

Fig. 31.—Diagram of theory of differential pattern in periodograph analysis.

If, however, the fringe takes some other slant, as the direction *ac*, making the angle *θ* with the analyzing lines, then the period desired is the time in years between *a* and *c*. That equals the time between *a* and *b* less the time from *b* to *c*. Now *bc* in years would equal except for the fact that the horizontal scale along *bc* is greater than the vertical scale along *ab* in the ratio and therefore a definite space interval along it means fewer years in the ratio of . Hence we have:

*bc*(in years) =

*ab*(in years) tan δ cot θ

or

_{1}(1 — tan δ cot θ)

which is the period required.

The separation of the fringes needs to be known at times in order to find whether one or more actual cycles are appearing in the period under test. In figure 31

which is the width required.