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

60 Manual of basic techniques for a health laboratoryIf the lamp is fitted with a lens, the filaments of the bulb are projected on to a pieceof paper covering the mirror. This makes it possible to centre the beam more precisely.In some models the bulb is turned until a clear image of the filament isobtained.Preliminary adjustment of the mirrorUse the plane side of the mirror. Remove any coloured filters. Open the iris diaphragmto the maximum. Raise the condenser. Place a piece of thin white paperover the lens at the top of the condenser (Fig. 3.15).The piece of paper should show an image of the electric bulb, surrounded by acircle of light. Adjust the mirror so that the image of the bulb is in the exact centreof the circle of light (Fig. 3.16). If daylight is being used, adjust the mirror so as tomaximize the amount of light passing through the condenser.Fig. 3.15 Adjusting the mirrorFig. 3.16 Image of the lightsource, as seenthrough thecondenserFig. 3.17 To centre thecondenser, first closethe diaphragmFig. 3.18 Raise the condenseruntil the edges ofthe circle of light arein focusCentring the condenser (if centring is provided for)It is very important to centre the condenser correctly. This is often overlooked.1. Place a slide preparation without a coverslip on the stage. Lower the condenser.Open the iris diaphragm. Examine with the lowest-power objective (¥ 3, ¥ 5 or¥ 10). Look through the eyepiece and bring the slide into focus.2. Close the diaphragm. A blurred circle of light surrounded by a dark ring appearsin the field (Fig. 3.17).3. Raise the condenser slowly until the edges of the circle of light are in sharp focus(Fig. 3.18).4. Adjust the position of the mirror (if necessary) so that the circle of light is in theexact centre of, or superimposed upon, the bright area surrounded by the darkzone (Fig. 3.19).5. Adjust the centring screws of the condenser so that the circle of light is in theexact centre of the field (Fig. 3.20). Then check with the other objectives.