526 Prof. Lovering on the Velocity of Light 



true path. The angular displacement which this causes between 

 the apparent and real places of a star is called aberration, and 

 was first discovered by Bradley in 1726, — this astronomer 

 explaining, on this simple principle, anomalies in observation 

 which had hitherto been considered accidental. As the dis- 

 placement of the star works opposite ways at opposite seasons 

 of the year, half the difference between the extreme places is 

 the distance from the apparent to the true place, or the constant 

 of aberration. This, when known as an observed fact, esta- 

 blishes the ratio between the velocity of light and the velocity of 

 the earth, and enables the astronomer to assign the value of 

 the one with all the accuracy which pertains to his knowledge 

 of the other. Accepting Struve's determination of the aberra- 

 tion, viz. 20"*45, the velocity of light is calculated to be 10,088 

 times as great as the velocity of the earth. The mean velocity 

 of the earth is known with all the certainty which belongs to our 

 knowledge of the magnitude of the earth's orbit, that is, of the 

 sun's distance. Assuming, as before, that the distance derived 

 from Encke's parallax is the most reliable, the velocity of the 

 earth in one second of solar time is 18*977 miles. This, multi- 

 plied by the aforesaid ratio, gives 191,513 miles for the velocity 

 of light by Bradley's principle. It appears therefore that the 

 velocities by the two methods of astronomy (the direct and the 

 indirect) differ by 1837 miles — a small quantity comparatively, 

 being only one per cent, of the whole velocity. Whatever other 

 value is adopted for the sun's distance will alter these two results 

 proportionally, without disturbing the ratio between them. I 

 may add that the velocity which aberration ascribes to light 

 belongs to it at the earth's surface — that is, in the dense atmo- 

 sphere ; whereas the velocity discovered from the eclipses is that 

 which extends through the planetary spaces. This distinction, 

 however, will do little towards bringing the two results into 

 greater accordance. The velocities of light in different media 

 are proportional to the indices of refraction inversely, which 

 in the case presented are as 1 to 1*000294. This theoretical 

 difference of velocities is less than ^V^ of the whole, or less 

 than 70 miles. 



. Compare with these conclusions of astronomy two experi- 

 mental results on the same subject. Although Wheatstone's 

 experiment on the velocity of electricity, published in 1834, 

 suggested the possibility of measuring in a similar way other 

 great velocities, I shall consider first a contrivance of Fizeau, 

 equally applicable to light and to electricity. If a wheel finely 

 cut into teeth on its circumference is put in rapid rotation, a ray 

 of light which escapes between two consecutive teeth will, after 

 feeing reflected perpendicularly by a mirror, return to strike the 



