The greatest error, excluding the one just mentioned, would probably be less than .00009 in the measurement of φ. Summing up the various errors, we find, then, that the total constant error, in the most unfavorable case, where the errors are all in the same direction, would be .00015. Adding to this the probable error of the result, .00002, we have for the limiting value of the error of the final result ±.00017. This corresponds to an error of ±51 kilometers. The correction for the velocity of light in vacuo is found by multiplying the speed in air by the index of refraction of air, at the temperature of the experiments. The error due to neglecting the barometric height is exceedingly small. This correction, in kilometers, is +80. Final Result. 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. [Illustration: FIG. 14.]