Illuminating Apparatus and Methods of Illumination

ILLUMINATING APPARATUS AND METHODS OF ILLUMINATION The methods of illumination commonly used are very varied; each method requires the proper apparatus for its convenient ap plication. The principles involved in some of the most impor tant of these, together with the apparatus needed, are here dis cussed.

Illumination by Transmitted Light.

To examine an ob ject in this way, light from a suitable source is passed through the object and so into the object-glass. Usually the light from the source is reflected from a mirror mounted below the object, the mirror being generally mounted so as to allow of a considerable range of movement both of translation and also of rotation. The mirror may be either plane or concave ; usually both types are provided, mounted back to back in a single mount. With a small source of light the plane mirror gives an illuminating beam com posed of practically parallel rays. With an extended source the rays reflected from the plane mirror reach the object along direc tions which give illumination over an appreciable angle. The con cave mirror can provide a convergent cone of illumination from quite a small source.

For obtaining an illuminating cone of larger angle than the con cave mirror can give, a condenser is interposed between the (plane) mirror and the object. The condenser is a system of lenses capable of concentrating the rays reflected from the plane mirror so that they converge on to the object as a cone of wide angle (see fig. 22). A diaphragm, usually of the "iris" type, is fitted below the condenser, in order that the angle of the cone of illumination may be varied conveniently at will. By focussing the condenser so as to form an image of the source, or of a bull's eye lens in front of the source, on to the object, intense illumina tion of the object is obtained.

Provision should be made for "centering" the condenser in order that the illuminating beam may be truly axial. Dry condensers capable of filling an object-glass having a numerical aperture of 0.95 are obtainable, well corrected for chromatic and spherical

aberrations. Immersion condensers are made with numerical apertures as high as 1.4, similarly corrected for chromatic and spherical aberrations and capable of being used with microscope slides of thicknesses up to about 1.5 mm. Immersion condensers are brought up close to the underside of the microscope slide, and are put into "immersion contact" with this by filling the space between the underside of the slide and the top surface of the condenser with cedar-wood oil. Immersion condensers are used when it is desired to fill the aperture of an immersion object glass.

Dark-ground Illumination.

If the condenser aperture is opened wide and a central circular stop is fitted below the con denser, the rays reaching the object form a hollow cone. A similar effect can be obtained by special methods of construction. By choosing an opaque central stop of suitable size, all the direct rays from the condenser can be made to pass outside the object glass, and no light will be received into the object-glass except that which is reflected, refracted, or scattered by the object so as to pass into the object-glass. If the object contains particles, veins, or other structure capable of di verting the rays into the object glass by any or all of these three processes, such inclusions or structure will be rendered visible in the microscope, and will be seen bright against the dark ground corresponding to the clear homogeneous portions of the ob ject and mounting medium.

Ordinary dry and immersion condensers can be used to give dark-ground illumination if pro vided with central opaque stops.

For work with object-glasses of apertures above o.65, immersion condensers fitted with central opaque stops can be used, but more satisfactory results are ob tained with special immersion dark-ground illuminators. The reflecting systems used in these illuminators give less stray light and at the same time produce a blacker ground.