A plastic light conduit of cross-linked polymer material having good light transmitting characteristics, without voids or noticeable bubbles, is disclosed. Also disclosed is a method of manufacture and an apparatus, involving progressive heating of reactive monomer mixture in a tube which is submerged in a progressive reactor utilizing cold oil to maintain a cold, non-reacting zone, and hot water to create a hot zone that causes reaction, while simlutaneously carrying away the excess exothermic heat of reaction.