to facilitate installation of the lamp the limiting column 61 of the cap 10 is typically first inserted into the limiting hole 62 of the housing 20 whereupon the tubular body 40 is fitted between the top cover 21 and bottom cover 22 of the housing 20 before assembling the top cover 21 and bottom cover 22 together so that the tubular body 40 is enclosed within the cylindrical cavity 50 when the tubular body 40 is in place inside the cylindrical cavity 50 the axis of the cylindrical cavity 50 coincides with the axis of the tubular body 40 which is also the axis of the cap 10 as the protrusion 45 of the tubular body 40 slides within one of the circular slots 52 relative rotation is enabled between the cap 10 and the housing 20 while the positioning members 47 48 also slide within the circular slots 52 corresponding to their positions and serve to increase friction during the rotation process when the protrusion 45 in the circular slot 52 slides to the point of intersection with the linear slot 51 relative telescopic motion is enabled between the cap 10 and the housing 20 and when the protrusion 45 slides in the linear slot 51 the positioning members 47 48 move across the plurality of circular slots 52 through an elastic deformation of the flexible portions 43 44 in addition after the cap 10 and the housing 20 are adjusted to a particular relative position the positioning members 47.