Opaque Object Microscopy part: III

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In the above may be seen one each of the two primary varieties of vertical illuminators. Please understand that neither is in working order, but for illustrative purposes only they will do nicely. They are each quite similar in application but functionally and mechanically enormously different. Illumination in each case is provided by a bulb holder that slips into the large heat sinks seen on each. In the above may be seen one each of the two primary varieties of vertical illuminators.

That on the right is the more traditional of the two, and will not be found overly different from any other vertical illuminator on offer from other makers of the same period. The apparatus could in theory be attached to a stand already possessed, provided the body tube featured a draw tube or short tube length objectives were employed. To that end the upper mount is outfitted with a male society thread. At the bottom in place of the usual rotating turret nose-piece is a bi-directional friction fitting. Objectives to be used must first be screwed into specialized which is then lifted into place and twisted to seat. With the heat sink removed one can observe the field condenser lens. There is a frosted glass surface facing the bulb and a convex polished surface on the obverse. Continuing down the line is a field diaphragm, followed by a filter holder, an achromatic doublet condenser lens, and an aperture diaphragm.

Within the body of the vertical illuminator is found the first reflecting surface, a circular half silver mirror. When observed from the ocular side it is all but invisible (above left). However, when observed from the specimen side its reflective capacity is immediately apparent (above right, green filter inserted). The large knurled heat located at 9 o’clock may be turned a few degrees in either direction to offset the reflector and so center the illumination. When pulled outwards the knurled head exposes a right angle prism from its recess. The prism obstructs nearly half the field of view but being a complete mirror provides excellent illumination.

The second vertical illuminator is of a type I consider metallurgical and requires specialty objectives (see previous post) rather than specialty objective mounting hardware as in the prior example. The metal heat sink is of the same size as the other vertical illuminator but the condenser body is significantly larger. This is owing to the light path that is needed for effective illumination. The field lens of the this condenser (that closest to the light source) is a bi-convex lens with the axial portion facing the light source ground rather than polished. The aperture side (or specimen facing if you prefer) lens is similarly ground at it’s axis but outfitted with an additional feature, a light-opaque axial filed stop. This stop may be moved into the light path to provide dark-field illumination. This is a particularly useful feature when observing metallurgical specimens as most are highly polished and reflective after having been etched.

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In the above image may be seen the light path of the illumination source. The brightly lit outer ring would correspond to a similar optical conductor in the objective. The dark axial portion would provide a path for the image forming rays to be conducted through the objective and into the body of the microscope thence through the ocular and into the eye of the viewer. In this way the light source for a metallurgical vertical illuminator has rather more in common with a Lieberkühn or ring light as employed on a stereo microscope. The dark field stop prevents the light from the illuminator from finding its way into the image forming axis except those rays which are reflected by the specimen. With the dark field stop removed the frosted central portions of the condenser lenses provide a uniform illumination that hides the image of the lamp filament which would otherwise appear in the image plain.

Next time: a highly specialized type of microscope for opaque object work. -K

 

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