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 especially, the conclusive experiments of Michelson and Morley, and Trouton and Noble, in which, a motion through the ether, of the earth in its path around the sun would certainly have been detected. For the purposes of this article we shall consider that the first postulate of relativity needs no further proof.

It is Einstein (to whom, indeed we owe the development of relativity along its present broad lines) who first stated the second postulate of relativity in a general form, namely, that the velocity of light in free space appears the same to all observers, regardless of the relative motion of the source of light and the observer. This is the assumption which has forced the theory of relativity to its

strange conclusions, and it is for its further consideration that this paper is designed.

A simple example will make the extraordinary nature of the second postulate evident.

S is a source of light and A and B two moving systems. A is moving towards the source S, and B away from it. Observers on the systems mark off equal distances aa' and bb' along the path of the light and determine the time taken for light to pass from a to a'  and b to b'  respectively. Contrary to what seem the simple conclusions of common sense, the second postulate requires that the time taken for the light to pass from a to a' shall measure the same as the time for the light to go from b to b'. Such a consideration makes the path obvious by which the theory of relativity has been led to strange conclusions as to the units of length and time in a moving system.

The second postulate of relativity is obtained by a combination of the first postulate with a principle which has long been familiar in the theory of light. This principle states that the velocity of light is unaffected by a motion of the emitting source, in other words, that the velocity with which light travels past any observer