with reference to fig1 a , a helmet shell 51 is shown from the left side of the user &# 39 ; s head ( the right side having symmetrical features ). a mounting rail 55 in accordance with the invention is included within a fixture preferably fabricated from nylon , polypropylene , or other synthetic plastic using injection molding processes , the bottom edge of which conforms to the bottom edge of the helmet shell 51 . the fixture is secured to the exterior of helmet shell 51 by means of fasteners 53 in the front and back . in another embodiment , illustrated in fig1 b , mounting rail 55 is included within a fixture having a bottom edge that conforms to a convex extension 56 of helmet shell 51 . a recessed groove 57 in the mounting rail 55 slidably accepts a complementary engagement member 59 of a potential accessory , which may be secured by tightening a securing member 60 ( e . g ., a standard screw , as illustrated , or a thumbscrew , tab - and - slot system , or other suitable engagement mechanism ). the engagement member 59 shown in fig1 a and 1 b is illustrative only ; in practice , it would carry a functional accessory . virtually any accessory suitable for mounting to the helmet 51 can be designed to have an engagement member complementary to the recessed groove 57 . in fig2 the mounting rail 55 is shown in cross - section , mounted on helmet shell 51 . in the illustrated embodiment , the cross - sectional profile of the recessed groove 57 is a dove - tail configuration complementary in cross - section to that of an attaching component 59 . the dovetail shape retains the attaching component 59 by means of the angled edges 61 , but allows it to slide within the recessed groove 57 to reach a desired position where it is further secured by a tightening screw 64 . the ends of the mounting rail 55 desirably slope toward the surface of the helmet shell 51 . as shown in fig3 a , the mounting rail 55 provides a geometrical interface between the curving surface of the helmet shell 51 and a straight groove 57 . a straight groove 57 is beneficial because it allows attaching component 59 to be formed with simple flat surfaces while still allowing adjustment of the position of the accessory along the length of the groove 57 . on the other hand , as shown in fig3 b , another embodiment of the mounting rail utilizes a non - straight recessed groove 67 . the inner surface of the mounting rail 65 has a contour that conforms to the helmet shell 51 , and the recessed groove 67 also approximates the curved surface of the helmet shell 51 but with a constant radius . the curved groove 67 has the benefit of reducing the protrusion at the ends of the mounting rail 55 shown in fig3 a . if the recessed groove 67 has a constant radius , the attaching component 59 can have a matching curved shape and still freely slide within the groove 67 . the mounting rail 65 provides an interface between differently sized helmets having different amounts of curvature and a groove 67 with a common constant radius ( independent of the size and curvature of the helmet ). for accessories that do not require the flat surface as shown in fig3 a , and thus the lower profile of mounting rail 65 shown in fig3 b , a curved mounting rail may be preferred . fig4 a shows the mounting rail of fig1 mounted on helmet shell 51 with two accessories 75 and 80 positioned to be attached . as illustrated , accessory 75 is an earphone ; a similar earphone can be mounted on the right side of the helmet in a symmetrical mounting rail ( not shown ). the earphone 75 is attached by a connecting member 77 to the engagement member 79 , which can itself be positioned along recessed groove 57 by sliding engagement member 79 therealong and securing it with the thumbscrew 83 . accessory 80 is an illuminator that can be similarly attached by sliding the engagement member 89 within recessed groove 57 and securing it with thumbscrew 83 . fig4 b shows the mounting rail of fig4 a with the two accessories 75 , 80 mounted in place . fig5 shows another embodiment of the present invention mounted to a helmet shell 51 . in this embodiment the mounting rail 55 on the left side of the helmet ( shown ) is extended forward by a front connecting element 94 , which joins with the mounting rail 55 on the right side of the helmet ( not shown ). the entire mounting rail thus encircles three - quarters of the helmet shell 51 and is secured by means of five fasteners 53 , two on each side and one in the front , which desirably penetrate the shell using the through - holes shared with retention components ( not shown ). this embodiment may comprise additional attaching features , it being understood that any particular version may have some , but not necessarily all of the attaching features illustrated . in addition to the recessed groove 57 already described , this embodiment has one or more threaded holes 97 , which serve as mounting points for an accessory that can be threadably mounted therein . an attachment surface 99 on the front connecting element 94 accepts accessories such as pvs - 14 night vision goggles to the front of the helmet . a hole 101 a on the left side of the front connecting element 94 can be used in conjunction with a similar hole 101 b on the right side to provide a hinged mounting point in the front for an additional accessory ( see fig6 ). finally , a slot 103 allows a rear connecting element 107 to be attached as further described in connection with fig7 . fig6 shows a face - protection accessory 105 hingeably affixed to the mounting rail of the present invention using holes 101 a and 101 b . the face - protection accessory 105 may be further supported by a bumper 107 that braces against the helmet shell 51 . mounting holes 101 a , 100 b provide a secure , hinged attachment to the helmet , allowing the face - protection accessory 105 to be hinged upward and out of the way when not needed . with reference to fig5 and 7 , a rear connecting element 107 is designed to mate with slot 103 a in mounting rail 55 on the left side and to extend around the back of the helmet shell 51 to a similar slot 103 b on the mounting rail on the right side of the helmet . the rear connecting element 107 is desirably slightly compliant and conformal with the outer shell 51 such that when a tension is established between the two slots 103 a and 103 b , the rear connecting element 107 comes into close contact with the shell 51 . this rear connecting element 107 , which creates a bridge under tension between left - side and right - side mounting rails , allows the rails to better resist dislodgement by horizontal or rotational forces . the tension may be established by a pair of over - the - center latches 110 a , 110 b , which have ends adapted to fit into slots 103 a , 103 b , respectively , and to move hingeably downward ( as indicated by the arrows ) so as to snap against connecting element 107 . alternatively , tension can be provided by other suitable means known in the art such as tightening screws that pull two parts of the rear connecting element 107 together to contract its length . accessories may be attached externally to the rear connecting element 107 using any of the attaching features as described above in connection with fig5 . alternatively , rear connecting element 107 can provide a protected space for smaller accessories , such as electronic components , which can be stored within the space within the rear connecting element 107 or between it and the outer shell 51 . having described certain embodiments of the invention , it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention . the described embodiments are to be considered in all respects as only illustrative and not restrictive .