| 摘要 |
674,265. Counting particles photo-electrically. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Jan. 9, 1951 [Jan. 10, 1950], No. 595/50. Class 40 (iii). In counting the number of particles randomly distributed on a surface such as in a diluted blood sample, different width strips are successively examined, a detector producing an electrical pulse in response to each particle. The invention is based on the assumption that the correct number of particles in unit area of a long strip of length y and width n is <SP>1</SP>/ y <SP>dn</SP>/ dc where n is the number counted. As shown, the sample in the form of a thin film in a recess on a glass surface 5 is traversed beneath the eyepiece of a microscope in such a way that a long strip is moved beneath the objective, the whole sample being then advanced so that a further strip is scanned. The light directed through the sample by a source 5 is received at a photomultiplier 9. The width of the strip under examination is determined by a rectangular aperture 7 which may be rotated through a right-angle in its own plane to vary the width. Traversing mechanism, Fig. 2.-The specimen on the surface 10 is advanced steadily against the action of a spring 18 in guides 13 by a cam 21 on the shaft of a continuously-driven motor 30 (Fig. 1). On the same shaft is mounted a bevel wheel 23 having only two teeth which advances the specimen carrier in steps through a leadscrew 28, and a commutator 31 which shortcircuits the photo-tube amplifier 33 at those times when the slide is not traversing uniformly. The commutator also operates a counter 36 which switches off the motor after the slide has advanced a certain number of times. In use the aperture 7 is set so that a wide strip is examined, 200 strips, say, being examined successively while the counter 34 counts the number of particles encountered. The aperture is now rotated and 200 narrow strips are examined, the drive to the register being reversed so that the number of particles is subtracted. In the arrangement described the aperture is 20 microns by 10 microns so that the final number of particles counted is the number in 200-10 micron strips. Additional counters 38, 39 preceded by pulse width discriminators may be used to count particles in particular size ranges. |
