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

62 Manual of basic techniques for a health laboratoryOccasionally a clear image cannot be obtained although the objective has beenlowered as far as possible. This is because the fine adjustment screw has been turnedright to the end. Turn it back as far as it will go in the other direction and then focusby raising the objective. Rack the condenser up slightly if there is insufficient illumination.High-power objective (¥ 40)Rack the condenser half-way down. Lower the objective until it is just above theslide preparation (the working distance is very short — about 0.5mm). Using thecoarse adjustment screw, raise the objective very slowly until a blurred image appearsin the field. Bring into focus using the fine adjustment screw. Raise the condenserto obtain sufficient illumination. If the microscope has no condenser, usethe concave side of the mirror.Oil-immersion objective (¥ 100)Perfectly dry, stained preparations must be used. Place a tiny drop of immersion oilon the part to be examined (use synthetic oils, which do not dry, in preference tocedarwood oil, which dries quickly). Rack the condenser up as far as it will go, andopen the iris diaphragm fully. Lower the ¥ 100 objective until it is in contact withthe oil. Bring it as close as possible to the slide, but avoid pressing on the preparation(modern objectives are fitted with a damper). Look through the eyepiece andturn the fine adjustment screw very slowly upwards until the image is in focus. Ifthe illumination is inadequate, use the concave side of the mirror as recommendedfor the ¥ 40 objective.Important: In most modern microscopes, it is not the objective holder but the stagewhich is moved up and down by the coarse and fine adjustment screws to bring theimage into focus.Depth of the microscope fieldThe image is seen in depth when a low-power objective is used. When the highpowerobjectives (¥ 40, ¥ 100) are used, the depth of focus is small and the fineadjustment screw must be used to examine every detail from the top to the bottomlevels of focus of the object observed (e.g. the different nuclei in a spherical amoebacyst).Images seen under the microscopeRemember that the circle of light seen in the eyepiece is called “the microscopicfield”.How to establish the position of images seenImages observed in the microscopic field can be placed in relation to the hands ofa clock. For example, a schistosome egg is placed at “2 o’clock” in Fig. 3.23.Inversion of imagesThe image seen is inverted by the lenses:●●Objects seen at the bottom of the microscopic field are actually at the top.Objects seen on the left side of the microscopic field are actually on the right.Fig. 3.23 Establishing theposition of imagesseen under themicroscopeMoving the objectIf you move the slide in one direction, the object examined moves in the oppositedirection (Fig. 3.24).