Illuminating Apparatus and Methods of Illumination

In Wenham's system the resolving power of the object-glass is halved, in so far as detail lying parallel to the short dimensions of the semicircular half-apertures is concerned. To avoid this, sys tems are made in which light from the whole object-glass aper ture is used in forming each of the images (fig. 29). Stereo old-fashioned "Lieberkiihn" but it is differently constructed, provides much more intense illumination, and is much more convenient.

The object to be examined is mounted on a microscope slide, and on the underside of the slide a small patch stop is gummed if necessary ; alternatively an opaque central stop of suitable size can be mounted below the microscope stage. A parallel beam of light is sent up through the stage, and reaches the lens-mirror as a hollow cylinder of light. Any light the object sends back into the object-glass has been either diffusely reflected or scattered by the object. What is seen represents the most natural appear ance the object could present if it were enlarged to a degree cor responding to the magnification given by the microscope.

By building a hollow condenser system round the object-glass, the firm of Chapman and Alldridge has produced a type of ver tical illuminator which gives the same kind of illumination as that given by the ring illuminator. A plane mirror is mounted above the condenser with its surface in clined at 45° to the axis of the combined system, an elliptical hole being cut in the mirror to allow of its being fitted round the object-glass. Through a hole in the side of the mount light is admitted. This light is reflected down through the condenser sys tem, and so is brought down to the surface of the object. By ad

justing the position of the light source, an image of this source can be brought to focus on the surface of the object at the same time as the microscope is in focus for the object. The appearance of the object resembles that obtained when a ring illuminator is used and, since all the illuminating apparatus is above the object, this type of illuminator is of value in metallographic work, or for use in the examination of any large opaque object. Like the ring illuminator, this illuminator enables "top" illumination to be obtained without incurring any of the difficulties associated with vertical illuminator glare.

Side Illumination.

It is sometimes necessary or useful to illuminate the surface of the object by light projected on to the object from one side and inclined to the surface at almost graz ing incidence. This can be done by using a powerful beam, con sisting of almost parallel rays, directed slightly downwards on the object. A more convenient way, if great intensity of illumina tion is required, is to mount a half-lens of high power on the stage of the microscope, so that the principal focus of the half-lens lies in the surface of the object and in the centre of the field of view of the object-glass. An alternative method avoids the production of a coloured image of the source, by a paraboloid reflector.

This method of illumination produces pronounced shadow effects on the surface of the object and shows any surface irregularities in sharp relief. The method is of value in studying the surface contours of objects if due care be taken in translating the appear ances seen in the image.