the preferred embodiments of the present invention will now be discussed in detail with reference to the accompanying drawings . according to the present invention , the flashlight comprises a tail cover , a housing body , batteries in the housing body , and a reflector , a lock device and a bulb holder which are assembled together and then mounted on the housing body . fig1 is the three - dimensional exploded view of the focusing flashlight according to the first embodiment of the present invention , wherein only the reflector 1 , the lock device 4 and the bulb holder are showed . as shown in fig1 , the reflector 1 comprises a parabolic lampshade and a barrel portion 10 integrally attached to the top end of the lampshade . the inner screw threads 11 are formed on the inner wall of the barrel portion 10 of the reflector 1 and the grooves 7 elongated along the axial direction of the barrel portion 10 are formed on the outer wall at equal intervals in the circumference direction . the grooves 7 do not penetrate the wall of the barrel portion 10 and the width of the grooves 7 is not uniform , with the portion of the grooves near the lampshade being relatively smaller and that near the free end growing larger . the portion with larger width can extend in the circumference direction to either one of or both sides relative to the narrower portion . the lock device 4 is a cylindrical part with an inside diameter approximately equivalent to the outside diameter of the barrel portion 10 of the reflector 1 . the protruding ribs 6 are formed in the axial direction on the inner wall of the lock device 4 at equal intervals to be engaged with the grooves 7 of the barrel portion 10 of the reflector 1 , and the height of the protruding ribs is approximately equal to the depth of the grooves 7 . the projections 14 project in the axial direction from the end of the lock device 4 away from the reflector 1 at equal intervals . the number of the grooves 7 formed on the reflector 1 is the same as or an integral multiple of the number of the protruding ribs 6 formed on the lock device 4 . the bulb holder comprises a bulb - holding base 5 and a bulb - inserting base 3 . the bulb - holding base 5 is formed in the shape of a hollow cylinder and has a flange on one of its ends . the bulb - inserting base 3 with bulb 2 installed therein is inserted into the hollow portion of the bulb - holding base 5 . the outer screw threads 12 are formed on the outer wall of the cylindrical portion of the bulb - holding base 5 to be engaged with the inner screw threads 11 formed on the inner wall of the barrel portion 10 of the reflector 1 , and a plurality of recesses 13 corresponding to the projections 14 of the lock device 4 are formed on the flange of the bulb - holding base 5 at equal intervals in the circumference direction . the number of the recesses 13 formed on the flange of the bulb - holding base 5 is the same as or an integral multiple of the number of the projections 14 formed on the lock device 4 . fig2 shows schematically the assembly of the reflector , the lock device and the bulb holder in fig1 ; fig3 is a sectional view taken along the line a - a in fig2 . in assembling , the protruding ribs 6 are inserted into the grooves 7 of the barrel portion 10 of the reflector 1 so that the lock device 4 is set around the exterior of the barrel portion 10 of the reflector 1 . and then , the bulb holder is mounted on the reflector 1 by screwing the outer screw threads 12 of the bulb - holding base 5 on the inner screw threads 11 of the reflector 1 . fig4 shows schematically the reflector , the lock device and the bulb holder which are mounted on the housing body of the flashlight . after the reflector , the lock device and the bulb holder are assembled , they can be installed integrally on the housing body of the flashlight to form a integrated flashlight . fig5 is the three - dimensional exploded view of the focusing flashlight according to the second embodiment of the present invention , wherein only the reflector , the lock device and the bulb holder are showed . the second embodiment is almost the same as the first embodiment , except that the positions of the protruding ribs 6 and the grooves 7 are varied . in the second embodiment , the protruding ribs 6 are formed in the axial direction on the outer wall of the barrel portion 10 of the reflector 1 at equal intervals , and the grooves 7 to be engaged with the protruding ribs 6 are formed on the inner wall of the lock device 4 at equal intervals in the circumference direction . the width of the grooves 7 is not uniform , with the end near the bulb holder being narrower and the end near the reflector 1 growing wider . during the focusing after the bulb 2 is installed , the lock device 4 coupled with the barrel portion 10 of the reflector 1 is firstly slid toward the lampshade of the reflector 1 so that the projections 14 of the lock device 4 are disengaged from the recesses 13 on the flange of the bulb - holding base 5 , and thus allow the bulb - holding base 5 to rotate . as the bulb - holding base 5 is rotated , the bulb 2 moves backwards and forwards with the bulb - holding base 5 under the guiding of the outer screw threads 12 of the bulb holder , so that the luminous point of the bulb 2 is coincided with the focus of the parabolic lampshade to gain the optimal focusing effects . subsequently , the lock device 4 coupled with the barrel portion 10 of the reflector 1 is slid toward the flange of the bulb - holding base 5 and the projections 14 of the lock device 4 are inserted into the recesses 13 on the flange of the bulb - holding base 5 . and then , the lock device 4 is slightly rotated to move the protruding ribs 6 away from the position aligning with the narrow portion of the grooves 7 . thus , the lock device 4 can not move to and fro on the reflector 1 and the positions of the bulb 2 and the reflector 1 can remain relatively fixed . fig6 is the three - dimensional exploded view of the focusing flashlight according to the third embodiment of the present invention , wherein only the reflector , the lock device and the bulb holder are showed . in the third embodiment , the shape of the grooves 7 formed on the reflector 1 is different from that in the first embodiment , and the grooves 7 are straight grooves with a uniform width . in addition , a spring 15 around the barrel portion 10 of the reflector 1 is set between the lock device 4 and the reflector 1 . the other structures of this embodiment are the same as the first embodiment . fig7 shows schematically the assembly of the reflector , the lock device and the bulb holder in fig6 . as shown in fig7 , the projections 14 of the lock device 4 are kept in the recesses 13 on the flange of the bulb - holding base 5 under the action of the spring 15 . fig8 is the three - dimensional exploded view of the focusing flashlight according to the fourth embodiment of the present invention , wherein only the reflector , the lock device and the bulb holder are showed . the fourth embodiment is almost the same as the third embodiment except that the positions of the protruding ribs 6 and the grooves 7 are different . in the fourth embodiment , the protruding ribs 6 are formed in the axial direction on the outer wall of the barrel portion 10 of the reflector 1 at equal intervals , and the grooves 7 to be engaged with the protruding ribs 6 are formed on the inner wall of the lock device 4 at equal intervals in the circumference direction . the grooves 7 are also the straight grooves with a uniform width . for the flashlights according to the third and the fourth embodiments of the present invention , in focusing after the bulb 2 is installed , the lock device 4 coupled with the barrel portion 10 of the reflector 1 is firstly moved toward the lampshade of the reflector 1 with overcoming the elasticity of the spring 15 , so that the projections 14 of the lock device 4 are disengaged from the recesses 13 on the flange of the bulb - holding base 5 , and thus enable the bulb - holding base 5 to rotate . as the bulb - holding base 5 is rotated , the bulb 2 will move forwards and backwards with the bulb - holding base 5 under the guiding of the outer screw threads 12 of the bulb holder , thereby the luminous point of the bulb 2 is coincided with the focus of the parabolic lampshade to gain the optimal focusing effects . then , the external force applied on the lock device 4 is released , and the lock device 4 moves toward the bulb - holding base 5 under the action of the elasticity of the spring 15 , the projections 14 of the lock device 4 is remained in the recesses 13 on the flange of the bulb - holding base 5 . and thus , the lock device 4 cannot move to and fro on the reflector 1 and the positions of the bulb 2 and the reflector 1 can remain relatively fixed . the present invention is not restricted by the above description . for instance , the said projections can be formed on the side of the flange of the bulb holder facing the lock device , while the said recesses can be formed on the said lock device . those skilled in the art can make a lot of variations in the present invention without departing from the scope of the claims of the invention .