referring now to the drawing in detail , it may be seen that the reference numeral 1 has been used to identify an apparatus equipped in accordance with the present invention in its entirety . the apparatus 1 is operative for collapsing a tubular formation 2 into a solid cross - section optical preform . the tubular formation 2 is supported in a holder or chuck 3 which , in turn , is mounted for relative rotation on a stationary support 4 . the end of the tubular formation 2 which is remote from the chuck 3 merges into an exhaust pipe 5 which , in turn , is rotatably mounted , by means of rotary union 6 of a conventional construction , on a stationary support 7 . during the collapsing operation , the chuck 3 rotates the tubular formation 2 , together with the exhaust pipe 5 , about the axis of the tubular formation 2 , while the rotary union 6 provides for a substantially leakagefree communication between the interior of the exhaust pipe 5 and the interior of the support 7 . a heating device 8 , such as an oxyhydrogen burner , is mounted for movement in the directions indicated by a double - headed arrow , along the tubular formation 2 substantially between the chuck 3 and the region of merger of the tubular formation 2 with the exhaust pipe 5 . a gaseous medium at a predetermined superatmospheric pressure is introduced through an inlet pipe 9 into the interior of the tubular formation 2 during at least some but preferably all but the last one , of the increments of the collapsing operation which is induced , in a well - known manner , by the heating action of the heating device 8 . as well - known in the art , the heating device 8 is moved along the tubular formation 2 during each of the collapsing increments or passes , resulting in incremental reduction of the inner and outer diameters of the tubular formation 2 , until the internal passage present in the tubular formation 2 is completely eliminated during the last collapsing increment and the tubular formation 2 is converted into a solid rod or optical preform . as described so far , the apparatus 1 is of a known construction as frequently used in the optical fiber manufacturing field so that it is deemed not to be necessary to describe this construction here in any greater detail . suffice it to say that the support 7 is hollow and is provided with a hollow connecting portion 10 to which there is connected , by means of a connecting arrangement 11 of any conventional construction , a connecting conduit 12 which is shown to be constituted by a flexible hose . the pressure prevailing in the interior of the hollow portion 10 is admitted to a conduit 13 which is shown to have two branches 14 and 15 . the branch 14 leads to a reference gauge 16 which is constructed , for instance , as magnahelic measuring device of a conventional construction . on the other hand , the branch 15 leads to a pressure sensor 17 which has an inlet port 18 through which ambient pressure is admitted into the interior of the pressure sensor 17 . the pressure sensor 17 is also of a conventional construction which will not be discussed here in any detail , and is operative for comparing the pressure supplied thereto by the branch conduit 15 with the ambient pressure entering the same through the port 18 , and for generating an electrical signal representative of the result of the comparison of these two pressures , preferably their difference . the pressure sensor 17 is preferably of the type known as a barocell pressure sensor and is mounted on a thermal base 19 in order to reduce or eliminate thermal effects on the electrical output thereof . tne electrical output of the pressure sensor 17 is supplied to a connecting line 20 which carries this output signal to an electronic manometer 21 . the electronic manometer 21 produces an output which is supplied , through a connecting line 22 , to a set point controller 23 . the end of the flexible hose 12 which is remote from the connecting arrangement 11 is connected to a port which communicates with the interior of a dead space cylinder 24 . the interior of the dead space cylinder 24 is partitioned , by a partitioning wall 25 , into two compartments 26 and 27 . the compartments 26 and 27 are in limited communication with one another via a restrictor 28 of a predetermined flow - through cross - section . an outlet port 29 communicates the compartment 27 with a discharge conduit 30 that leads to a discharge valve 31 that is capable of controlling the flow through the discharge conduit 30 and discharge of the gaseous medium present in the conduit 30 into the ambient atmosphere . the valve 31 is advantageously constructed as a pvc butterfly valve . a feeding conduit 32 which has an adjustable valve 33 interposed therein communicates with the compartment 26 of the dead space cylinder 24 . an inert gas , such as , for instance , nitrogen , is admitted into the conduit 32 . a connecting line 34 connects an output of the set point controller 23 with the adjustable valve 33 . the valve 33 is adjusted , in dependence on the magnitude of the signal received from the connecting line 34 , so as to control the flow of the inert gas through the conduit 32 into the compartment 26 . having so described the construction of the apparatus 1 according to the present invention , its operation will now be briefly discussed . during the collapsing operation , a gaseous medium at a predetermined pressure is admitted into the interior of the tubular formation 2 through the intake pipe 9 . simultaneously , the inert medium , especially nitrogen , is supplied into the compartment 26 of the dead space cylinder 24 . depending on the degree of opening of the valve 33 , pressure will build up in the compartment 26 , this pressure build - up prepagating through the flexible hose 12 and the tubular portion 10 of the support 7 into the interior of the exhaust pipe 5 . of course , a part of the contents of the compartment 26 will simultaneously be discharged through the restriction 28 into the compartment 27 and from there through the discharge port 29 , the discharge conduit 30 and the discharge valve 31 into the ambient atmosphere . nevertheless , the afore - mentioned pressure build up will still take place . the pressure sensor 17 detects the excess of this built - up pressure over the ambient pressure and issues an output signal into the line 20 which is fed into the electronic manometer 21 and from there through the connecting line 22 to the set point controller 23 . then , when the pressure prevailing in the tubular portion 10 deviates from that desired , the set point controller 23 issues a signal into the line 34 which causes the valve 33 either to close more or to open more depending on the sense of deviation , such that the throttling effect of the valve 33 is increased when the deviation is upwardly of the desired pressure , and is decreased when the deviation is downwardly of the desired pressure . simultaneously , the gauge 16 provides a visual indication of the pressure inside the tubular portion of the support 7 , enabling the operator of the arrangement 1 to change the values pre - set in the set point controller 23 , if need be . it will be appreciated that the dead space cylinder 24 acts as a pulsation damper in that temporary fluctuations will be dissipated therein . on the other hand , long term trends in the pressure build - up will be detected by the pressure sensor 17 and will be used to control the setting of the flow - through cross - section of the valve 33 . obviously , the pressure in the tubular portion 10 is always below that prevailing in the intake pipe 9 , so that flow is established through the tubular formation 2 during the collapsing operation in the direction from the intake pipe 9 to the exhaust pipe 5 and from there through the tubular portion 10 , the flexible hose 12 , the compartment 26 , the restriction 28 , the compartment 27 , the discharge port 29 , and the discharge conduit 31 into the ambient atmosphere . it was established that , during the collapsing operation , certain dopants , such as germanium , phosphorus or the like , evaporate more readily than the basic silicon material of tne tubular formation 2 . usually , the innermost layer previously deposited in the interior of the tubular substrate to form the formation 2 is the most heavily doped one , since it is desired to have the greatest deviation in its index of refraction from that of silica . hence , the evaporation of such dopant from this innermost layer results in deterioration of the desired optical properties of this layer and / or several outwardly adjacent layers . to counteract this deleterious occurrence , the medium admitted into the interior of the tubular formation 2 through the intake of pipe 9 during the collapsing operation is of that type which releases the respective dopant or dopants when exposed to heat . this , of course , means that , during the collapsing operation , some of the dopant material will evaporate from the innermost layer and be carried away by the flow of the gaseous medium through the interior of the tubular formation 2 but , at the same time , replenishment amounts of the dopant are formed in the gaseous medium at the region of the heating device 8 inside the tubular formation , these additional amounts of the dopant depositing on the internal surface of the tubular formation 2 downstream of the region of the heating device 8 . hence , the depletion of the dopant or dopants from the innermost layer of the tubular formation 2 is at least partially compensated for . in any event , the loss of the dopant or dopants from the radially outwardly adjoining layers , which would result from the diffusion of the dopant into the innermost layer and evaporation from the latter , is minimized if not eliminated altogether . according to the present invention , the pressure control described above is preformed during all of the passes of the heating device 8 , that is , during all of the collapsing increments which change the tubular formation 2 into a solid cross - section optical preform , except for the last collapsing increment during which the internal passage of the originally tubular formation is eliminated . it will be appreciated that , because of such elimination of the internal passage of the tubular formation , at least locally , at the very outset of the last collapsing step , no flow of the gaseous medium can take place through the interior of the tubular formation 2 and , consequently , the very purpose of the above - discussed pressure control would be defeated . the dead space cylinder 24 controls the short - term pressure fluctuations . however , experience has shown that , in many instances , the dead - space cylinder 24 and the associated elements can be eliminated without deleteriously affecting the desired build - up of the back - up pressure in the exhaust pipe 5 , in that the conduit 32 leads into the interior of the tubular formation 2 and terminates there to cause back - up pressure at this location . experience has shown that sufficient control of the back - up pressure is obtained even under these circumstances . while we have decribed above the principles of our invention in connection with specifice apparatus , it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of our invention as set forth in the objects thereof and in the accompanying claims .