Manual of basic techniques for a health laboratory - libdoc.who.int

3. General laboratory procedures 55— 0.30 on the ¥ 10 objective— 0.65 on the ¥ 40 objective— 1.30 on the ¥ 100 objective.The greater the numerical aperture, the greater the resolving power (see below).Moreover, the greater the numerical aperture, the smaller the front lens mountedat the base of the objective. The front lens of the ¥ 100 objective is the size of apinhead, so handle it with care.Other figures that may be marked on the sleeveThe sleeve may also display:— the recommended length in millimetres of the tube (between the objectiveand the eyepiece) — usually 160mm;— the recommended thickness in millimetres of the coverslip used to cover theobject slide — e.g. 0.16mm.The screw threads of all objectives are standard, so the objectives in the revolvingnosepiece are interchangeable.Working distanceThe working distance of an objective is the distance between the front lens of theobjective and the object slide when the image is in focus. The greater the magnifyingpower of the objective, the smaller the working distance (Fig. 3.5):— ¥ 10 objective: the working distance is 5–6mm— ¥ 40 objective: the working distance is 0.5–1.5mm— ¥ 100 objective: the working distance is 0.15–0.20mm.Fig. 3.5 Working distance of an objectiveResolving powerThe resolving power of an objective is its ability to reveal closely adjacent details asseparate and distinct. The greater the resolving power of the objective, the clearerthe image.The maximum resolving power of a good medical laboratory microscope is about0.25mm (the resolving power of the normal human eye is about 0.25mm).Immersion oil increases the resolving power by conserving many light rays thatwould be lost by refraction if a dry objective were used.