Relativistic addition of perpendicular velocity components from the   constancy of the speed of light
  Mermin [Am. J. Phys. {\bf 51}, 1130--1131 (1983)] derived the relativistic addition of the parallel components of velocity using the constancy of the speed of light. In this note, the derivation is extended to the perpendicular components of velocity. 
Ben Yu-Kuang Hu