[Illustration: Fig. 201.--Bull’s-eye Lens.]
To illuminate opaque objects the light should be thrown upon them from above by the bull’s-eye lens (Fig. 201). The focus of such a lens and the lamp placed at four inches from it, is about three inches for daylight, or two inches for artificial light. A large object may be placed upon the stage of the microscope at once, but smaller objects are either laid on a glass slide or held in the stage forceps.
When illuminating objects from above all light from the mirror, or that which might enter the objective from below the stage, should be carefully excluded. _Dark-field illumination_ is a means of seeing a transparent object as an opaque one. The principle, however, is that all the light shall be thrown from below the object, but so obliquely that it cannot enter the object-glass unless interrupted by the object; this is best accomplished by _Wenham’s Parabola_.
_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.