in fig3 and 5 the positioning device 54 serves to position the filament of the bulb 42 in five dimensions namely along the z axis which is parallel to the plane of fig1 that is it runs the length of the apparatus illustrated in fig1 along the y axis which is perpendicular to the z axis but is also in the plane of the paper bearing fig1 along the x axis which is orthogonal to the z and y axes angularly about the x axis which may be termed u03b8x and angularly about the y axis which may be termed u03b8y the x y and z axes are indicated in fig1 to 6 to aid understanding positioning along the z axis is achieved by a slideway 56 and a screw 58 working between a part 60 associated with the track 56t of the slideway 56 and a part 62 associated with the carriage 56c of the slideway 56 positioning along the x axis is achieved by screws 66 which extend through slots 64 extensive in the x direction through base 70 positioning along the y axis is achieved by shims 68 between the track 56t and the base 70 and between the part 60 and the base 70 the base 70 is fixedly secured to the housing 22 by means not illustrated the orientation about the x axis is held i e u03b8x is held by clamping screws 72 u03b8x is selected after slackening of the screws 72 by rotation in a pinion 74 secured to shaft 76 rotatably mounted in trunnion 78 the pinion 74 cooperates with a rack 80 to drive the holder 52.