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 the reflected beam the ordinary velocity of light as from any stationary source. If the surface of the grating should act in this way, then the reflected light would have the same velocity whichever of our hypotheses were true. Moreover, except for second-order terms, both hypotheses have led to the same conclusions as to the frequency of the light, and since the wave-length of light is completely determined by its frequency and velocity, such an action of a reflecting surface would prevent our distinguishing between the two hypotheses. We can, however, draw one useful conclusion from our consideration, namely, that if the velocity of light does depend on the velocity of the source, then a reflecting mirror acts as a new source, and the velocity of the reflected beam depends only on the motion of the mirror.

Before proceeding to the consideration of an experiment in which we shall make use of the principle just derived, let us consider whether a transmission grating would also act as a new source of light and destroy any original difference between the velocities of light, for example, from the two limbs of the sun. In the next section we shall consider more in detail certain experiments of Fizeau and of Michelson concerning the velocity of light in media which are moving with respect to the source of light. We are, however, led by them to the conclusion that an original difference in the velocity of light from the two limbs of the sun would be only about one half obliterated by passage through a plate of glass, and hence the use of transmission gratings for deciding between the two postulates as to the velocity of light would be possible. At the present time very excellent transmission gratings are obtainable, being replicas of original Rowland gratings. It is desirable that observations be made with apparatus in which the light suffers no reflection in order to definitely settle the matter.

Experiments On The Velocity Of Light From The Two Limbs Of The Sun.
The fact that a mirror acts as a new source of light led me to the construction of a very simple apparatus for comparing the velocity of light from the approaching and receding limbs of the