EHRLICH-TYPE VARIABLE APERTURE EYEPIECE FOR DETERMINING RATIO OF WHITE TO RED CELLS IN A DRY BLOOD SMEAR.

CHARLES BAKER

MODEL: EHRLICH EYEPIECE

ENGLISH

c. 1926

The eyepiece is inscribed on the wider knurled black disk: EHRLICH EYEPIECE with the Baker insignia (C Baker, London) and the number 1mm followed by 2,3,4,and 5. These register against a single white line on the fixed central part of the eyepiece. The top portion of the eyepiece slides in and out of the eyepiece housing to allow precise focusing on the square aperture. The size of the square aperture is varied by turning the knurled disk while the position of the eyepiece is fixed to the optical tube by the knurled screw. The aperture was set to a smaller area for counting red cells and then opened to a larger area for counting white cells, which are found in much smaller numbers in the blood. Since the area in each square is known, the number of cells per unit area could be corrected to a common denominator and the ratio of red to white cells calculated. The normal ratio of red to white cells in blood is about 500 to 1. But abnormal ratios can occur due to changes in either the WBC count or the RBC count; this makes this ratio very difficult to interpret unless at least one of the two absolute counts is known.

HISTORY:

This device, reported in the Glascow Medical Journal of May 1901, volume 55, pages 339-40 was apparently initially produced by Leitz. In the Leitz catalog it was referred to as an Aperture Eyepiece.

The device was apparently used for decades, offered by Charles Baker in their 1926 catalog. This device could be used to determine the ratio of red to white cells but not absolute counts per volume which required a hemocytometer slide. Today blood counts are done by automated machines on liquid blood, though hemocytometer slides are still used for counting cells in other body fluids like spinal fluid. The Ehrlich eyepiece is no longer needed.