4. Fit up the source of illumination some 30-50 cm. distant from the microscope. (This should be the Liliput Arc Lamp (Leitz), Nernst Lamp or incandescent gas lamp; if either of the two latter are employed, a bull's eye condenser to produce parallel rays must be interposed between light and microscope); and adjust illuminant and microscope so that the substage plane mirror is completely filled with light. 5. Focus the two concentric rings engraved upon the upper surface of the condenser and centre them accurately by means of the centring screws. 6. Prepare a "fresh" specimen (see pages 74-76) of the material it is desired to observe, using selected, new, 3 by 1 glass slips of less than 1 mm. thickness, and No. 1 cover-glasses (0.17 mm. thick), which should be cleaned with a piece of soft washleather and not with the emery paper, as scratches on the glass produce haziness in the preparation. 7. Deposit a large drop of immersion oil (or pure water) on the upper surface of the condenser and rack it down a few millimetres. 8. Adjust the fresh preparation on the microscope stage and fasten it in position with the stage clips. 9. Rack up the condenser until the immersion fluid makes contact with the under surface of the slide; avoid the formation of air bubbles. 10. Adjust the substage mirror so that the light is reflected upward. A bright spot will be seen on the fresh preparation near the centre of the field. 11. Replace the 2/3-inch objective by the 1/12-inch oil immersion lens which has been fitted with the special stop to reduce its N. A.; place a drop of immersion oil upon the centre of the cover-glasses of the fresh preparation and lower the microscope tube until the front lens of the objective has entered the oil drop. 12. Focus the bright spot referred to in step 10. If it no longer occupies the centre of the field, alter the angle of the substage mirror until it does. 13. Now focus the lens accurately on the film, cautiously vary the height of the dark ground condenser until the best position is found. The intensely illuminated bacteria will stand out in vivid contrast to the dark background. [Illustration: FIG. 70.--Immersion oil bottle.] ~Microscopical Examination of the Stained Specimen.~--(The body tube of the microscope may be vertical or inclined to an angle.) 1. Secure the slide on the stage of the microscope by means of the spring clips. 2. Place a drop of cedarwood oil on the centre of the cover-slip. The immersion oil is pure cedarwood oil, and is kept in a small bottle of stout glass (Fig. 70), the cavity of which is shaped like an inverted cone, and is provided with a safety funnel (so that the oil does not escape if the bottle is accidentally overturned) and a dust cap of boxwood fitted with a wooden rod with which the drop of oil is applied to the cover-glass or lens.