referring in detail to the drawings the reference numeral 11 designates an enclosure developed by a fixed frame structure including horizontal and vertical members 12 and 13 , respectively . the enclosure is completed by glazing assemblies g which comprise one or more panels of glazing materials supported in the frame members with appropriate gaskets to make the unit weather tight . in order to facilitate transition from one glazing assembly to another , performing all work on the exterior of the enclosure without disturbing personnel or material within the enclosure , a novel mullion device m is provided in combination with a vertical member 13 of the enclosure 11 . referring in detail to fig3 and 4 a mullion device m is shown secured to a vertical member 13 receiving at the left side a glazing assembly 14 of cross - sectional thickness t 1 making a transition to receive at the right side to a glazing assembly 16 having a smaller cross - sectional thickness t 2 . for reasons which will become more apparent as the specification proceeds the glazing assembly 14 is termed a &# 34 ; secure &# 34 ; assembly and the glazing assembly 16 is termed &# 34 ; non - secure &# 34 ;. the secure assembly 14 includes typically bullet resisting glazing materials not necessarily limited to glass but may include laminated sheets or assemblies of glass , polycarbonates , metals and other resinous materials as security risks dictate . the secure assemblies may be transparant or opaque . in the embodiment disclosed in fig4 the secure assembly 14 includes inner plates 17 and 18 clad with outer plates 19 and 21 all fabricated of polycarbonate resin compounds . the non - secure glazing assembly 16 comprises ordinary glazing materials 22 and 23 . the sills supporting the glazing assemblies are of sufficient structural integrity to support heavy glazing assemblies ranging in weight from 3 . 2 to 12 . 6 pounds per square foot of glazing material whereby non - secure glazing installations may be replaced by secure installation without the necessity to rebuild or strengthen the load bearing sills . as shown in fig4 the mullion device m cooperates with vertical member 13 in that a vertical channel 26 defined by an inner sidewall 27 and a separable outer sidewall 28 provide a receptacle for glazing assembly 16 . note that the inner sidewall 27 of channel 26 defines a planar surface of vertical member 13 while the outer sidewall 28 is connected to a leg 29 to define an l - shape . glazing assembly 16 having a thickness t 2 plus conventional peripheral gaskets 31 and 32 does not create a &# 34 ; pile up &# 34 ; sufficient to fill the channel 26 . consequently , a rigid filler element 33 , coextensive with inner sidewall 27 and held in place by spaced segments of adhesive means 34 , is inserted to fill out the required pile up . the filler element 33 extends vertically and horizontally about the perimeter or edge margin of the non - secure glazing assembly 16 . the vertical member 13 is formed with a rib 36 against which a projection 37 of leg 29 bears as it is bottomed on inner sidewall 27 . the leg 29 includes a bridge 38 and fastening means in the form of a machine screw 39 which makes a connection with the rib 36 to secure leg 29 to vertical member 13 . while leg 29 is shown connected to and spaced from a similar leg 41 by bridge 38 it is entirely within the contemplation of the invention to fabricate each leg as separate piece parts with individual fastening means connecting bridge portions to the rib 36 . it is preferable that the legs 29 and 41 be joined by bridge 38 to define a single piece part to facilitate changing glazing assemblies as will be more apparent hereinafter . for purposes of claiming the invention the unitary piece part having joined legs 29 and 41 is referred to as a removable glazing clip . leg 41 bounds channel 42 having an inner sidewall 43 and outer sidewall 44 . the glazing assembly 14 having thickness t 1 plus gaskets 46 and 47 adequately fill the cross - sectional dimension of channel 42 without the need for a filler element . an elongated cover 48 closes the gap 49 that exists between legs 29 and 41 . the cover includes flexible projections 51 and 52 terminating in eared tabs 53 and 54 which effect a camming action while overriding and interlocking with mating lugs 56 and 57 from on the legs 29 and 41 , respectively , to secure the cover . the cover may be structured in a continuous fashion as an extrusion or consist of a plurality of contiguous cover segments , as desired . in the alternative embodiment shown in fig9 the cover 148 is a solid bar which interlocks with legs 29 and 41 in the fashion previously described . in the fig9 arrangement the cover 148 provides bullet resistant protection &# 34 ; backed up &# 34 ; by a thickened wall 149 on the vertical member 130 . in this fashion the mullion m of fig9 provides a bullet resistant joint between bullet resistant glazing panels without the need to resort to expensive packing material such as is shown at b in fig4 through 8 . furthermore , the modification illustrated in fig9 provides protection against projectiles and other missiles impacting the wall structure at oblique angles as shown by the arrows and legends in fig9 . in addition , the bullet resistant wall 149 provides additional ballistic security in situations where , through human error , packing material b ( fig6 ) is not present . obviously , the elimination of the ballistic packing b and its replacement with bullet resistant cover 148 and bullet resistant wall 149 reduces labor costs and eliminates the cost of the packing material as well . a further consideration , which the glazing system of the present invention anticipates , is the size limitations of ballistic glazing relative to conventional glazing sizes . transparent ballistic glazing costs of the order of 30 times the cost of conventional 1 / 4 inch plate glass and stock sizes of ballistic glazing are limited . thus , it is important that original non - secure , convertible glazing installations are designed to receive stock sizes of transparent ballistic plate to avoid extremely expensive special order ballistic plate . thus , the concept of the present invention includes a frame structure which is calculated to receive ( in a conversion from non - secure to secure plate ) stock sizes of transparent ballistic plate . in order to change an existing non - secure glazing assembly such as the assembly 16 , on the right side of fig4 to a secure glazing assembly such as exists on the left side of fig4 the following steps are practiced working from the outside of the enclosure : the cover 48 ( or 148 ) is machined to destruction by drilling , metal sawing or the like . thereafter a suitable wrench , such as a socket wrench , is used to back off the several fastening means 30 which are spaced vertically along the bridge 38 . this step releases the outer sidewalls 28 and 44 and the unitary glazing clip is removed . similar steps are undertaken at the opposite end ( not shown ) of the non - secure glazing assembly 16 to release the opposite end of the assembly . next the non - secure assembly 16 is removed ; the filler elements 33 at the perimeter of the assembly 16 including the adhesive means 34 are also removed . the assembly 16 is removed by lifting the assembly out of its sill and moving it into the head space or clearance provided in the upper horizontal frame member in well known fashion . gasket 31 ( or a new one ) is moved into keying relationship with inner sidewall 27 and a secure glazing assembly is moved into place . the legs 29 and 41 , as a unitary piecepart , are replaced straddling the rib 36 . upon setting up the fastening means 39 and installing a new cover 48 and reinstalling peripheral gasket 32 , the glazing assemblies of fig4 now appear as shown in fig6 with secure assemblies on both sides of the mullion device m . the language &# 34 ; non - secure &# 34 ; is intended to denote a panel or glazing assembly which will not repel bullets , similar projectiles or forced entry , while the language &# 34 ; secure &# 34 ; is intended to denote bullet repellant or bullet resistant panels or glazing assemblies , as well as assemblies to prevent forced entry . thus it is apparent that the present invention provides a mullion device which is operative to receive a first non - secure panel or glazing assembly in a permanent or temporary installation while lending itself to conversion to a different assembly secure or non - secure , where the assemblies may have a wide variety of thicknesses . it is merely necessary to select a filler element of appropriate dimensions to fill out the channel dimension as required . in secure installations such as shown in fig6 it is necessary to make the peripheral frame members secure as well . one way to secure the frame members is to introduce bullet resistant packing b ( fig6 ). another way is to increase the gauge or thickness of the various horizontal and vertical extrusions comprising the frame members as shown in fig9 . to effect a transition from one glazing assembly to another in the field , a kit is provided which may include new gaskets , new cover filler element consistent with the width of the channel and the thickness of the glazing assembly , or other parts needed for individual requirements . fig8 shows a modified removable glazing clip in which leg 41 having outer sidewall 58 includes an endwall 59 . clips of this configuration are used next to a jamb , or wall configuration . it is understood that the invention is not limited to the illustrations described and shown herein , which are deemed to be merely illustrative of the best modes of carrying out the invention , and which are susceptible of modification of form , size , arrangement of parts and details of operation . the invention rather is intended to encompass all such modifications which are within its spirit and scope as defined by the claims .