The fact that time stops for it, and the fact that it is the limiting velocity, give to $C$ certain of the attributes of the mathematician's infinity. Certainly if it can never be exceeded, we must have a new formula for the composition of velocities. Otherwise when my system passes yours at a speed of 100,000 miles per second, while yours passes a third in the same direction at the same velocity, I shall be passing this third framework at the forbidden velocity of 200,000 miles per second--greater than $C$. In fact Einstein is able to show that an old formula, which had already been found to connect the speed of light in a material medium with the speed of that medium, will now serve universally for the composition of velocities. When we combine the velocities $v$ and $u$, instead of getting the resultant $v+u$ as we would have supposed, we get the resultant $$(v + u) / (1 + (uv / C^2))\,,$$ or $$C^2(v + u) / (C^2 + uv)$$ This need not surprise us either, if we will but reflect that the second velocity effects a second revision of length and time measurements between the systems involved. And now, if we let either $v$, or $u$, or even both of them, take the value $C$, the resultant still is $C$. In another way we have found $C$ to behave like the mathematician's infinity, to which, in the words of the blind poet, if we add untold thousands, we effect no real increment. ASSUMPTION AND CONSEQUENCE A good many correspondents who have given the subject sufficient thought to realize that the limiting character of the velocity $C$ is really read into Einstein's system by assumption have written, in more or less perturbed inquiry, to know whether this does not invalidate the whole structure. The answer, of course, is yes--provided you can show this assumption to be invalid. The same answer may be made of any scientific doctrine whatever, and in reference to any one of the multitudinous assumptions underlying it. If we were to discover, tomorrow, a way of sending signals absolutely instantaneously, Einstein's whole structure would collapse as soon as we had agreed to use this new method. If we were to discover a signalling agent with finite velocity greater than that of light, relativity would persist with this velocity written in its formulae in the place of $C$.