We all know this book... its is basically the SR as we know it. you can find it here ww.fourmilab.ch/etexts/einstein/specrel/www what i was looking for was how and on what basis back on 1905 the Albert figures out the speed of light is constant... all I get is this "Any ray of light moves in the ``stationary'' system of co-ordinates with the determined velocity c, whether the ray be emitted by a stationary or by a moving body. Hence velocity=light path/time interval where time interval is to be taken in the sense of the definition in § 1." I might not have red it through very carefully, but i spent an evening examining and reading it diagonally. It doesn't mention any experiments that prove the quotation, it also doesn't offer any thought experiment that could help us see that the c is realy constant. is there any document writen by Albert or any other matematician or is there any experiment that proves the light speed constancy (before publishing this book - 1905)??? This is very important to me because lorentz transformation and all the formulas I been calculating mass,time, lenght,energy etc. is based on the consumtion that c is constant.