the carriage 32 downwards and towards the zone 6 of overlap the rod 30 is mounted so that it can slide vertically along the carriage 32 as depicted diagrammatically by the arrows 38 the mount 34 extends the rod 30 downwards this mount 34 is mounted so that it can rotate on the rod 30 about the axis x u2014 x of the rod 30 as depicted diagrammatically by the arrows 40 the structure 14 bears the optical coupling means 19 and the connecting means 21 the head 18 and the nozzle 22 which surrounds the head 18 are coaxial with the rod 30 and with the mount 34 the axis x u2014 x of the rod 30 is substantially orthogonal to the zone 6 of overlap and the head 18 and the nozzle 22 are orientated towards this zone 6 the head 18 and therefore the nozzle 22 are mounted so that they can slide vertically on the mount 34 as depicted diagrammatically by the arrows 42 while the pressing mechanism 24 is secured to the mount 34 thus the head 18 and the nozzle 22 can be moved up closer to the zone 6 of overlap independently of the pressing mechanism 24 as illustrated more particularly in fig2 the pressing mechanism 24 comprises a hollow tube 44 with a circular base which is coaxial with the rod 30 and which surrounds the head 18 and the nozzle 22 this tube 44 borne by the mount 34 extends the latter downwards and therefore towards the zone 6 of overlap the lower edge face 46 of this tube 44 is equipped with an open faced annular runway 48 in which.