caps 7 are engaged in the recesses 6 with their tops facing outwards the installation further comprises a second annular track 11 with which a second rotary platform 12 is associated as mentioned above the rotary platform 12 is mounted to turn about an axial direction of the annular track 11 in this embodiment the platform 12 comprises an annular portion 13 overlying the annular track 11 and that is in contact with the caps 7 in order to cause them to turn about a radial direction of the annular track 11 on the inner side relative to a radial direction of the annular track 11 the caps are held by a lip 14 projecting relative to the annular track 11 and by a lip 15 projecting downwards relative to the annular portion 13 of the platform 12 on the outer side the caps are held by a slide 9 supported by the work surface by means of arms not shown the two platforms 5 and 12 turn in opposite directions in order to make transfer possible via the guide rails 9 and 16 in the transfer zone the lip 8 is interrupted in the embodiment shown the outer guide rail 16 is also interrupted after the transfer zone but it could be configured to extend perpendicularly to the track 11 and to provide internal guidance for the caps it should be observed that because of the opposite directions of rotation of the platforms and because transfer is direct from the first platform to the second platform the faces of each cap that are subjected to radiation is swapped over during transfer this makes it possible to treat both faces of the cap with a.