Page:Collected Physical Papers.djvu/48

28 When this is the case the receiver ceases to respond. Let A be the corresponding reading of the platform index. A stationary index I′ is now placed opposite the reading A of the graduated circle.

When the cylinder is rotated in the opposite direction a second reading B for the critical angle is obtained. It is obvious that, neglecting errors, A—B is equal to twice the critical angle.

The platform index is now clamped and the circle as a whole is rotated till B comes opposite to the fixed index I′. The circle is now clamped, the platform arm unclamped, and the central table rotated till another reading C for the critical angle is obtained. Then, as in the previous case, $$B-C=2i$$, where $$i$$ is the critical angle. The circle as a whole is now rotated till C comes opposite the fixed index.

Thus at each successive operation the circle is rotated past the fixed index through $$2i$$. The successive difference of readings of the circle in reference to the fixed outside index, thus gives a series of values of $$2i$$.

The result will be more accurate if we take the mean readings $$\tfrac{1}{2}(A+B), \tfrac{1}{2}(B+C)$$, ...., and take their differ-