I've just started reading the dustiest book on relativity i could find at the local library and like most others i've seen it includes the Michelson Morley experiment. I just need a bit of clarification on the formulae they use. The equations t1=2l/(c^2 - v^2)^1/2 and t2=2lc/c^2 - v^2 seem logical enough but i don't understand why the first one applies to the velocity of light perpendicular to the ether. The book i'm reading gets the equation from the situation of a boat of speed c crossing a flowing river of speed v. For the boat to travel perpendicular to the banks it has to drive slightly upstream so that its resultant velocity acts in the right direction - that being (c^2 - v^2)^1/2; if you're answering my question you've probably seen this. If the boat were to instead just aim straight in the direction of the opposite river bank, the river would cause it to travel diagonally; though it's horizontal motion would be completely independant of its vertical motion. Isn't this the same situation as with the perpendicular path of the light through the ether? So why do we apply the equation t1 to the velocity of the light? Why don't we just used the speed c?