_Glass_ of any kind requires occasional cleaning; a piece of soft washed chamois leather should be used for this purpose. The fronts of the objectives may be carefully wiped, but not _unscrewed_ or tampered with; a short thick-set camel’s hair brush may be passed down to the back lens, and all dust removed without doing any harm. If the objective is an _immersion_, carefully remove the fluid from the front lens, as even distilled water will leave a stain behind. For removing oil see special directions given at page 171. When cleaning the _eye-pieces_, which should be done occasionally, the cells containing the glasses must be unscrewed and replaced one at a time, so that they may not be made to change places. Any dirt upon the _eye-pieces_ may be detected by turning them round whilst looking through the instrument; but if the _object-glasses_ are not clean, or are injured, it will, for the most part, only be seen by the object appearing misty. The _object-glasses_, when in use but not on the microscope, should be stood upon the table with the screw downwards, to prevent dust getting into the lenses, and they should always be put into their brass cases when done with. A large bell-glass shade will be found the most useful cover for keeping dust from the instrument when not in use. When looking through the eye-piece be sure to place the eye in close proximation to the cap, otherwise the whole field will not be perfectly visible; it should appear as an equally well-illuminated circular disc. If the eyelashes are reflected from the eye-glass, the observer is looking upon the eye-piece, and not through it. _The Mirror._--The working focal distance of the mirror is that which brings the images of the window-bars sharply out upon the object resting upon the stage. In other words, the focus of the mirror is that which brings parallel rays to a correct focus on the object-glass. If employing artificial light, then the flame of the lamp should be distinguishable; a slight change in the inclination of the mirror will throw the image of the lamp-flame out of the field.