Illuminating Apparatus and Methods of Illumination

Most immersion dark-ground illuminators are made for a fixed working distance and are suitable for use with microscope slides of only one particular thickness. The illuminator can be made usable with a thinner slide by packing cover-glasses between the top lens of the illuminator and the underside of the slide, each cover-glass being covered on both sides with immersion oil to pro vide immersion contact between the top of the illuminator and the slide. If the slide is too thick there is no remedy. To avoid the inconvenience of a fixed working distance, the firm of R. and J. Beck has introduced an adjustable immersion dark-ground il luminator which, by moving the constituent optical elements rel ative to each other, can be made to work with microscope slides of any thickness between o.5 mm. and 1.5 mm.

A dark-ground illuminator computed by E. M. Nelson has the special feature that it will give dark-ground illumination with an object-glass having a numerical aperture as high as 1.4. The working distance of this illuminator is sufficiently large to permit of its being used with slides o.8 1.4 mm. thick.

Transmitted Polarized placing a Nicol's prism, or other polarizing appar atus, between the mirror and the condenser, the object can be il luminated by a cone of light in which the vibrations are all paral lel to one direction. If the object has any properties which cause it to modify the state of polarization of the light coming through the polarizer and condenser, these modifications can be observed, provided a second polarizing prism (the analyser) is used in con junction with the eyepiece.

If the analyser is set with its polarizing axis at right-angles to that of the polarizer, no light should be seen in the eyepiece, unless portions of the object have the power of modifying the state of polarization of the illuminating beam. For the complete examination of an object by polarized light, it is necessary to be able to rotate the polarizer and analyser together while keeping the object at rest, or else to rotate the object between the sta tionary prisms. It is also necessary to be able to rotate one of the prisms relatively to the other. Accessory apparatus of various types is convenient for the special examination of many kinds of objects which show polarization effects, such as fibres and crystals. Microscopes specially equipped with polarizing prisms and acces sory apparatus for the examination of objects by polarized light are generally described as petrological microscopes (see fig. 25).

Vertical Illuminator.

For examining opaque objects it is necessary to send light down on to the object from above. One method of doing this is to mount a cover-glass in a fitting above the object-glass, the plane of the cover-glass being inclined at to the axis of the microscope. Light, admitted through a hole at the side of the fitting, is reflected by the cover-glass down into the object-glass and is brought to focus on the object by the object glass. The light returning into the object-glass from the object is transmitted with but little loss through the inclined cover-glass, and proceeds to form an image in the usual way. Any inequalities in the cover-glass will tend to impair the resulting image, and in any case this image will be slightly astigmatic owing to the ref rac tion of the light through the inclined cover-glass. To render these effects as small as possible a thin uniform cover-glass should be selected for this purpose.

Prisms, semi-circular mirrors and other types of reflectors are used sometimes instead of the inclined cover-glass just described. Most of these give illumination through only one half of the object-glass aperture, and permit of only the other half being used to form the image of the object. Illumination in this way is apt to cause shadows on the object, while the use of only half the lens aperture causes the resolution to be reduced to one-half, in so far as detail lying parallel to the short dimension of the used aperture is concerned. Furthermore, the portion of the lens used to form the image has a semi-circular aperture which causes the diffraction rings seen round any image detail to depart from their usual circular form. All of these effects may give rise to misleading appearances in the image and may lead to the draw ing of false deductions as to the form of the object structure. To avoid these effects, some firms make small annular reflectors for vertical illuminator work. These reflectors send the light down through the outer zone of the object-glass and permit of the rest of the object-glass being used to form the image of the object. Such reflectors do not give rise to misleading effects of the type described above, they merely cut down the available numerical aperture of the object-glass by a small amount. The vertical illuminator is largely used for metallographic work and for ex amining large opaque objects.