| The mean value of V from the tables is | 299852 |
| Correction for temperature | +12 |
| ------------ | |
| Velocity of light in air | 299864 |
| Correction for vacuo | 80 |
| ------------ | |
| Velocity of light in vacuo | 299944±51 |
The final value of the velocity of light from these experiments is then—299940 kilometers per second, or 186380 miles per second.
Objections Considered.
Measurement of the Deflection.
The chief objection, namely, that in the method of the revolving mirror the deflection is small, has already been sufficiently answered. The same objection, in another form, is that the image is more or less indistinct. This is answered by a glance at the tables. These show that in each individual observation the average error was only three ten-thousandths of the whole deflection.
Uncertainty of Laws of Reflection and Refraction in Media in Rapid Rotation.
What is probably hinted at under the above heading is that there may be a possibility that the rapid rotation of the mirror throws the reflected pencil in the direction of rotation. Granting that this is the case, an inspection of Fig. 14 shows that the deflection will not be affected.
In this figure let m m be the position of the mirror when the light first falls on it from the slit at a, and m′ m′ the position when the light returns.
From the axis o draw op op, perpendicular to m m and to m′ m′, respectively. Then, supposing there is no such effect, the course of the axis of the pencil of light would be a o c mirror c o a′. That is, the angle of deflection would be a o a′, double the angle p o p′. If now the mirror be supposed to carry the pencil with it, let o c′ be the direction of the pencil on leaving the mirror m m; i.e., the motion of the mirror has changed the direction of the reflected ray through the angle c o c′. The course would then be a o c, mirror c′ o. From o the reflection would take place in the direction a″, making the angles c′ o p, and p′ o a″ equal. But the angle c o c′ must be added to p o a″, in consequence of the motion of the mirror, or the angle of deviation will be a o a″ + c o c′; or a o a″ + c o c′ = d. (1)