CI = ------------ ; (3)
cos squared[alpha]

[TEX: CI = \frac{CD}{\cos^2 \alpha}]

or, again,

___
AC squared
CI = ----- (4)
CD'

[TEX: CI = \frac{\overline{AC^2}}{CD'}]

11. In order to avoid all calculation, we may proceed thus: If I wish to
arrange the instrument so that C I represents a given quantity (Sec. 8),
I take (Fig. 7) the length Ci = CI/n, where n is any entire number
whatever.

[Illustration: Fig. 7.]

In other terms, Ci is the reduction to the scale of CI.

I describe the circumference C b i a, and arrange the instrument as seen
in the figure, and measure the length C b.

It is visible that

C i 1 C b C d
----- = --- = ----- = ------; then C B = n.C b (5)