Patent GB-714350-A (GB)
Title: Improvements relating to the counting of particles
IPC: G06M11/00
Filed: 1951-11-02
Abstract: 714,350. Photo-electric particle counters. MEYER, E. W. Nov. 3, 1952, [Nov. 2, 1951], No. 25647/51. Class 40 (3) In apparatus for counting small particles such as blood corpuscles a sample of the particles is scanned by a magnifying arrangement the image of the particles being received on detecting devices situated behind two apertured masks, the first of which has an aperture only slightly wider than the magnified image of the largest particle to be detected and the second of which has an aperture the width of which is much less than that of the other aperture, the difference of the counts of the detecting devices being an indication of the total number of particles scanned. Optical arrangements.-Light from a stabilized source 1 is directed by an optical system 2, 3 through a mechanically operated scanning stage 4 carrying a slide 5 on which are the particles to be counted. An enlarged.image of the sample is produced by a microscope 6 and falls upon t half-silvered mirror 9 or a Swan cube 9' Thus part of the light is transmitted to a photo-cell 12 and part reflected to a cell 15. Both cells may be of the multiplier type. Cell 12 is situated behind an aperture 10 of size slightly larger than that of the largest particle to be detected. Cell 15 is situated behind an aperture 13, the width of which is only a small fraction of that of the other aperture ; one-twentieth is suggested ; but the image received by it is immediately adjacent an edge of the zone received by image 10. The zone scanned by the narrow aperture should be adjacent to that scanned by the first aperture. The output pulses of the two photo-cells are fed through D.C. amplifiers 16, 17 to amplitude discriminators 18, 19. Discriminator 18 may be set for any particular size of particle but discriminator 19 transmits only pulses corresponding to complete obscuration of guard aperture 13. The amplitude discriminators feed through a gate circuit 20 which transmits only the excess of pulses from discriminator 18 over discriminator 19 to a register 21 of known type. Scanning arrangements (Figs. 3, 5).- The specimen is mounted on a slide 5 carried on an upper platform 22 which platform rests on a lower platform 24, steel balls 27 allowing relative movement between the two platforms. The line scanning movement is effected by means of a horseshoe magnet 39 which acts on an armature attached to the underside of the upper platform. Thus when it is desired to scan a specimen a motor 60 is switched on which then rotates a cam 70 carrying an eccentric pin 51. This pin engages a pillar 49 attached to pivoted bars 41 which also support the horseshoe magnet 39. Thus the magnet oscillates in a horizontal plane and moves the scanning table. The scanning table is advanced at the end of each line scan by notches 80 in cam 70 which through cam follower 72, lever 73, shaft 74, levers 75 and 76 and bell crank lever 77 actuate a pawl 78 to step around a ratchet wheel 79 and advance a screw 35 and thereby the lower carriage 24. A restoring spring 33 and limit microswitch are provided.