786,885. Photo-electric particle counting apparatus. MULLARD RADIO VALVE CO., Ltd. Feb. 23, 1954 [March 23, 1953], No. 7985/53. Class 40 (3). [Also in Group XL (c)] In apparatus for counting and sizing particles in which the scanning beam scans the sample with a rectilinear raster and in which the particle is explored on its first or last interception means are provided for " remembering" where a particle occurs in one line so as to distinguish between the first (or last) interception of a particle and any other interceptions. As shown, Fig. 1, a rectangular raster produced on the screen of a cathode-ray tube 1 by time bases 8 synchronized by a signal on the first channel of a magnetic recording track 9 is focused by a lens 3 on to the sample 4 the transmitted light being received by a photo-electric pick-up 6. If the sample is opaque reflected light may be used. A portion of a sample with three scanning lines L1, L2, L3 is shown in Fig. 2. Operation is as follows. As the raster traverses particle A a pulse is produced by the pick-up 6 which is fed through the pulse limiter and shaping circuit 7 (V1) to switches S1, S2, both in their left-hand positions. Switch 1 passes the pulse to a recording head 11 where it is recorded in the third track of a rotating magnetic record 9 which performs one half revolution in the time of one line scan. Switch S2 passes the pulse to a " new signal indicator 12 (V1).the purpose of which is to determine, by receiving pulses from reading head 13 spaced around the magnetic track, whether this is the first traverse of the particle. If it is the first pulse signal indicator 12 passes a pulse to switch control unit 14 and to pulse producing unit 15. Upon receipt of the pulse, unit 14 opens switches S1, S3, S4, S5 and S9 which (1) isolates the recording heads 11 and 13 from the photo-electric pick-up (2) arrests the line and frame scans (3) isolates erasing head 16. The unit &c. also passes a pulse to the signal routing switch 17 and the explore switch 18. The explore switch operates switch S6 so that the beam traverses the particle in the " Y " direction. The pulse producing unit 15 also energized by the output of the new signal indicator 12 passes a signal via switch S8 which signal is recorded on the second channel of the magnetic record by a head 20. The pulse producer also operates a sync. switch 21 which changes switches S7 and S8 to their left-hand positions so that subsequent signals read by head 20 will be passed via switches S7 and S8 to switch control unit 14. The pulse recorded by head 20 ensures that scanning always recommences when the magnetic record is in the same angular position as it was when scanning was arrested. When the vertical scanning of the particle is completed an output pulse is produced by pickup 6 which now passes direct to the explore switch 18 (since S2 is in its left-hand position). The explore switch arrests the vertical scan of the particle (by switch S6) and closes S7. Thus when the synchronizing pulse recorded on the second track is read by head 20, a pulse passes through switches S7 and S8 to switch control unit 14 and signal routing switch 17. Unit 14 is reset closing switches S1, S5 and S9 and opening S3, S4 so that normal scanning recommences. Routing switch 17 is also reset so that switch S2 passes any new signals from the limiter to new signal indicator 12. The pulse from head 20 is also fed to sync. switch 21 to reset switch S8. The closing of switch S9 causes erasing head 16 to erase the synchronizing signal from the second channel. Units 12, 14, 15, 17, 18 and 21 are now in their starting positions while the third recording channel carries a record of particle A. After scanning particles B, C and D the third channel carries a record of all four interceptions. During the scanning of line L2 the output signals from reading head 13 are fed to the new signal indicator at the same instant as the scanning spot has travelled the horizontal distance to where the particle was located in line L1. Thus if the particle does not overlap line L2 a pulse is received by unit 12 from head 13 but not from pick-up 6. If the particle does overlap the unit receives pulses from both 6 and 13. In neither case does this register as a " new signal " to operate the switch control unit. While line L2 is being scanned L1 is erased from the magnetic memory by head 16. Information as to the size of the particle is derived from the Y direction shift 19 which, at the end of the vertical exploration of a particle discharges a condenser previously charged to an extent depends on the length of the exploration period through resistor 22. The voltage pulse this produces indicates by its amplitude the size of the particle and a train of such pulses may be analysed into groups of various sizes as well as yielding a count of the total number of particles present. Details of the various circuits used are given below. The limiter circuit 7 consists of a cathodecoupled pair of triodes, the grid of the first triode being supplied with the photo-cell output pulse. The anode of the first triode is coupled to the grid of the second through a resistor and the circuit constants are such that a pulse over a predetermined amplitude results in the production of a rectangular pulse of predetermined amplitude at the anode of the second valve. The new signal indicator, Fig. 4, is an anticoincidence circuit designed to give an output pulse when a differentiated signal comprising a positive leading pulse followed by a negative pulse is applied to the diode V1 provided a positive pulse is not substantially simultaneously applied to the grid of V5. As shown, the positive leading pulse is applied to the anode of diode V1 the cathode of which is connected to the grid of a triode V2 acting as a pulse inverter. The negative pulse from the anode of V2 changes the state of a bi-stable multivibrator V3, V4 by blocking normally conducting valve V3 and rendering V4 conductive. The positive signal to diode V1 is also applied to the suppressor grid of V4. The negative pulse following the positive one cannot pass diode V1 and so restores the multivibrator to its original state by blocking valve V4. Momentarily much of the current of this valve is taken by the screen grid which now produces a negative output pulse. If a positive signal is simultaneously applied to the grid of V5 that valve conducts holding the grid of V4 negative and preventing the multivibrator from flipping. The switch control unit comprises a cathodecoupled double triode valve with the anode of each valve connected to the grids of the other and forming a bi-stable multivibrator. Negative signals applied to the first valve trip the circuit rendering the second valve conductive and producing a pulse which may be used to operate the explore switch while the anode circuit of second valve may include a winding of the multi-contact relay. The circuit may be restored by a negative pulse applied to the grid of the second valve. The signal routing and explore switches may be of the same type as the switch control unit while the mechanical switches may be replaced by electronic switches. The pulse producer is simply a mono-stable multivibrator circuit which is triggered by an input pulse to produce an output pulse and which returns to its original state after a predetermined time. Specifications 741,471, 747,718, 749,447 and 785,356 are referred to.