reflections along the light path the art of drawing glass fibers and glass tubes is quite diverse and well known and the present invention contemplates use of any of the known methods for heating a zone of a preform glass tube to permit drawing the preform tube into a tube of small diameter and appropriate length any other method such as drawing from an aperture in the bottom of a crucible is contemplated within the scope of this invention if it produces the desired final tubular line configuration by way of example one method for practicing the invention is illustrated in the sequence views of fig1 and 2 wherein a thick walled preform glass tube 20 is shown with an annular narrow zone 20 a subjected to a source of heat as represented by the arrows h a metal rod 21 placed centrally in the preform tube 20 is located in a relationship to the heated annular zone 20 a to permit heat re radiated from the inside wall of the preform tube to effect controlled melting of the tip of the metal rod during the drawing of the preform tube into its final small diameter form the rod 21 may be of a metal such as silver or aluminum to have a melting point substantially below that of the glass preform tube 20 typically for the arrangement ilustrated in fig1 thhe rod 21 may have a diameter of 0 001 to 2 mm.