connected to the output shaft of the motor 24 comprises a disc which incorporates a bevel which is sectored so as to be in two portions 186 and 188 as can be seen from fig1 each bevel portion 186 and 188 extends at an angle of 45 u00b0 to the plane of the disc 18 which has a matt white surface finish the motor 24 is a dc motor which has a built in tachometer and the speed of the motor 24 is controlled by a control circuit 202 as shown in fig2 by means of a potentiometer 204 which can be operated by the subject who can thus set the speed of rotation of the shutter 18 in the range of 300 to 2 400 rpm the potentiometer 204 sets a voltage v sp which determines the speed of the motor 24 the circuit 202 compares v sp with a feedback voltage from the tachometer v fd and generates a driving voltage v dr to drive and maintain the shutter 18 at the desired speed a viewing box 190 is located immediately adjacent the disc 18 and includes an aperture 192 to which light from the beams generated by the lamp 1 is incident on an achromatic magnifying lens 194 of the eye piece 2 the aperture 192 contains cross hairs the box 190 also contains a pair of incandescent lamps 196 and 198 for illuminating the white background provided by the interior of the box at the same brightness as the aperture.