*of various Substances for the Electric Ray.*

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 = 2*i*, 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 2*i*. The successive difference of readings of the circle in reference to the fixed outside index, thus gives a series of values of 2*i*.

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.