Opaque Object Microscopy part: II

Before looking closely at microscopes which were purpose built for reflected light work it is first imperative that one understands the requirements of such a stand. It must of course allow for some means of illumination, but beyond that there are a few needs that are not so obvious. Consider the compound transmitted light microscope, several aspects of it’s construction are dictated by the optical properties of human vision, a substantial number of others are dictated around changeable constants that are functionally arbitrary. A microscope slide and cover slip that is of a given thickness greatly simplifies the construction of an optical system that will provide an ideal image with minimal and known defects. Furthermore, it dictates that all specimens will be of a consistent and narrowly variable thickness.

Any microscope that needs to accommodate opaque objects will either have to account for the need to examine specimens of unknown thickness or be considered specialized. It need not be overly complex, one could make use of a stand having a significant range of motion in its coarse focus, or possessing a means of modifying its base working distance-as one finds on many stereo microscopes.. A further option would be to articulate the stage such that it may further enlarge the accommodation of the coarse focus. This is a simple mechanical alteration to an otherwise standard microscope foot; as a condenser is unnecessary, the stage is for all intents and purposes mounted to the condenser mount. An inverted microscope forgoes this need entirely by radically re-configuring the entire apparatus, a good investment if only one microscope is liable to be acquired, but it’s worth noting that inverted microscopes are in general far less common on the second hand market and consistently more expensive.

It is also required that the microscope provide for the specialized optics of a reflected light system, namely the light source. In the initial post we saw the difficulty of using a light source external to the image forming optical axis. It is therefore required that the illumination system be congruent with the optical axis. This requires that there will be some additional apparatus placed somewhere in the optical axis, by convention it is generally placed outside of the body, between the nose-piece and the end of the body tube. Whether this is dictated by optics rather than mechanics (or the economics of manufacturing) is unimportant, the result is the same.

At a point in the optical column of the microscope a high intensity light source is introduced. In every example of which I am aware this light source is situated perpendicular to the axis. It is suitably condensed and often fitted with a pair of iris diaphragms (a field diaphragm and condenser diaphragm) as well as filter carrier before being directed down towards the objective via a reflecting surface. A prism or half silver mirror is the usual method; often both are available with the ideal choice being contingent upon specimen and objective.

The actual construction of the reflector is related to the properties of the objective, with all parts involved being of a number of mechanical types. All of the differences in the system of illumination are chiefly concerned with the path of light. There exist two primary types: coaxial and vertical. It’s confusing because few operators, and even manufacturers are careful with their terminology.

Both coaxial and vertical illumination are methods of reflected light microscopy, and coaxial is by definition vertical while vertical is not necessarily coaxial. Coaxial illumination is so called because the path of the light source shares its axis with the path of the image forming rays. The poor mans coaxial illuminator is a flashlight held to one eyepiece of a binocular stand while the other is used for viewing-authors note: don’t do it! The axis of illumination and image formation are one and the same. Vertical illumination is a story of two axes where each is distinct but parallel. The most common type of vertical illuminator is able to provide both methods, but the quality of the coaxial illumination is often inferior when compared to modern outfits designed for coaxial illumination.

Without bothering to get in to dark-field (yes, there are dark field objectives for reflected light work) there are two types of objective one will encounter. The first is essentially no different from any standard objective, excepting of course for differences in the common powers, corrections, and other properties best left for later. The second, and more complex type, is designed to work with a particular type of light source. This second type (which I have always thought of as metallurgical as that is how the BalPlan microscope line of B&L designated them and they were the first I used) caries within its body a transparent glass bushing which extends from the mount to the object lens, surrounding totally and supporting the lenses of the objective. This glass pipe is little more than a means of placing a ring of evenly diffused light on the specimen in a place where the objective itself would obscure other sources. Properly arranged, it is an excellent system and dispenses with much of the glare one will find in poorly aligned coaxial or even vertical systems.

There, next time: photos! -K

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