# Page:MichelsonMorley1886.djvu/1

Consider the prism AC in motion relatively to the ether in direction AB with velocity ${\displaystyle \theta }$. Suppose the density of the external ether to be 1 and of the ether within the prism, ${\displaystyle 1+\Delta }$. In the time dt the prism will advance a distance ${\displaystyle \theta dt=BB}$. At