The angle of incidence is thus gradually increased, till the rays just undergo total reflection. 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 ½(A + B), ½(B + C), ...., and take their differences. Successive readings are taken till the graduated circle is rotated as near as possible through 360°.
As has been said before, there are two semi-cylinders P and Q. In the first set of experiments P is turned towards the radiator, Q acting as a focussing lens. The circle at each successive operation moves in a right-handed direction.
In the second set of experiments Q is turned towards the radiator, P acting as the converging lens. Successive readings are taken as before, the circle now rotating in a left-handed direction.