Page:Elementary Text-book of Physics (Anthony, 1897).djvu/418

404 falls on $$m,$$ is reflected, when $$m$$ is in a suitable position, through the lens $$L,$$ and forms an image at $$S'.$$ $$S$$ and $$S'$$ are conjugate foci of the lens, and by so placing the lens that $$S$$ shall be a little beyond the principal focus, $$S'$$ may be removed to as great a distance as desired. The mirror $$m'$$ is perpendicular to the axis of the lens, and at such a distance that the image $$S'$$ falls upon its surface. It is evident that any light reflected back from $$m'$$ through $$L$$ will return to the conjugate focus $$S,$$ whatever the position of the mirror $$m',$$ so long as it sends the light in such a direction as to pass through $$L$$ both going and returning. If now the mirror $$m$$ be given a rapid rotation clockwise, light passing through $$L$$ will return to find $$m$$ in a changed position, and the image will be displaced from $$S$$ to some point $$S$$ to the left of $$S.$$ Knowing the displacement $$SS$$ and the number of rotations of the mirror per second, the time required for light to pass from $$m$$ to $$S'$$ and back is determined. The value of the velocity of light, as determined by Michelson in 1879, is 299,910, and in 1882, 299,853, kilometres per second.