besides the disconnectable connecting means 28 and elastic means constituted by the spiral or torsion spring 40 the retraction control device for the mirror 10 comprises a tubular member 72 made from shape memory material and which is located between the end fittings 48 and 52 around the tie 46 and within the tubular portion 30 of the first part 24 the memory material member 72 is dimensioned in such a way that its length does not prevent the joining by friction of the parts 24 and 26 by means of the truncated cone shaped surfaces 58 and 62 on the one hand and 60 and 64 on the other when the nut 54 is tightened and when the mechanism is at a normal operating temperature however when the temperature of the memory material member 72 rises above a predetermined temperature threshold e g between 60 u00b0 and 100 u00b0 c the dimensioning is such that the elongation of said member e g by approximately 1 mm has the effect of moving the end fittings 48 and 52 sufficiently apart to eliminate the joining by friction of the parts 24 and 26 in order for the separation of said parts to take place correctly it should be noted that the connections by friction normally ensured between the complimentary truncated cone shaped surfaces 58 62 on the one hand and 60 64 on the other are not self locking in order that the elongation of the memory material member 72 has the effect of spacing apart the end fittings 48 52 fig1 shows that the tie 46 has a reduced resistance region 74 the region 74 can be constituted by a zone in which the diameter of the tie 46 is sufficiently reduced to permit its elongation when a.