surface structure of the specimen ic carrying the sought after bit pattern g t in phase the output signal of the comparator cmp is recorded dependent on the measuring point being impinged by the primary electron beam pe the output signal of the comparator cmp is used for modulating the intensity of the write beam of the picture tube crt which is deflected synchronously with the primary electron beam pe for a simple registration the write beam is deflected so that all interconnects which carry the sought after signal g t are displayed brightly for better localization of these structures within the highly complex circuit it is advantageous to superimpose a topographic signal such as a conventional secondary electron image of the specimen ic on the output signal of the comparator cmp so that they are displayed simultaneously in fig1 this is represented schematically by a signal line ts the only thing that the invention as set forth hereinabove will check is whether a defined bit pattern f t appears in phase with a sought after signal g t at an investigated measuring point it is possible to adjust the phase at which the test is undertaken by the use of a time delay element dl connected between the controller con and the clock input clk of the shift register sr since only the phase difference is of interest in such test the time delay element dl can instead be connected between the controller con and the specimen ic in fig2 is shown an arrangement of the present invention in which the test can be made phase independent in other words the test is undertaken to see whether the anticipated bit pattern is present at all.