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.
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.
The strongest light is reflected from the concave side of the mirror, that from the flat side is more diffuse and less intense. Oblique light can be obtained by turning the mirror on one side and then adjusting it so as to illuminate the field from that position. All the necessary mechanism of the microscope is easily and quickly learned. The object-glasses or objectives are, as previously explained, designated according to the focal distance of a single lens of the same magnifying power. Thus a 2-inch objective is understood to be a combination which has the magnifying power of a single lens whose focal point is two inches from the object, and so on with reference to other powers. By the aid of different eye-pieces an extensive range of magnifying power can be obtained; for example, the 2-inch objective with a deep eye-piece will give the same amplification as the quarter objective with the ordinary eye-piece. Indeed, for certain observations, the combination of a wide-angled low-power objective, with a deep eye-piece, or compensating eye-piece, is considered to have an advantage.