at 60 hertz this period is equivalent to more than 380 u00b0 of rotation of the scan in the absence of an output from and circuit 29 that is an absence of coincidence of the signals h and j the monostable 30 will time out causing a signal to be applied to terminal 31 which controls a relay which in turn is used to slow down the equipment since failure to produce a coincidence in and circuit 29 indicates that there has been a deviation of the pattern in excess of set number of degrees for example 21 6 u00b0 as shown in fig3 it will therefore be seen that photo sensor 15 senses rapid deviations of the pattern and produces in conjunction with the normal scan a slow down signal which can be used to control the tangential velocity of the tracer fig5 shows the scanner of fig1 with an additional feature to assist in operation a light source 48 provides light which is filtered by filter 49 and focused by lens 50 reflected by mirror 51 down the steering axis of the scanner through lens 16 and onto the pattern at point p3 which represents the intersection of the steering axis and the pattern by selection of the source 48 and the filter 49 a suitably contrasting light spot is projected onto the pattern at p3 the resulting light is selected to.