Opaque Object Microscopy part: I

I think at this point I’ve been away long enough that this may qualify more as a return from the dead than a return from a hiatus. -K
Most of what anyone at the level of a hobbyist is going to be looking at with a microscope is going to be what is convenient. Now, this is not meant as an indictment, merely a truthful commentary. For the bulk of those with a microscope this is going to mean that what one is going to be looking at is dictated first, by the microscope which is available, and only after by ones interest. When conditions allow this translates to transparent objects for the compound microscope and large opaque objects for the stereo microscope. There are, thankfully, limitless opportunities for the indulgence of ones interest regardless of the microscope which is available.

The next few posts will focus on a category of microscope which is rather less common but is specialized for a particular variety of specimen. The particular type of microscope is rather less common, and one could speculate endlessly on the reasons for this. This microscopist is of the opinion that the reason for this is in general attributable to its being far harder to prepare specimens for a reflected light microscope, than to settle for lower magnification and use a stereo microscope. However, there are a variety of applications for which one will find the power of a stereo microscope lacking. One is then left with the prospect of attempting to so treat the specimen as to render it suitable for transmitted light microscopy, or of finding some way of providing suitable lighting and using a standard compound microscope. Anyone who has attempted to observe an opaque object at high power will understand the difficulty of providing for adequate illumination.

For the sake of completeness, here are the logistics when one is forced to make use of a standard compound light microscope for reflected light work. One might first make use of some small and high intensity light source, employing it in such a way that the termination of the visual optical axis is brightly lit. This is actually surprisingly simple in the present day when an LED flashlight the size of a shotgun cartridge is brighter than any oil lamp. After oil lamps gave way to electric lamps the microscopist was required to somehow retrofit a standard lamp bulb so that it would provide a bright beam of light with no errant brightness.

There was also the possibility of purchasing a small bright light source such as a Nicholas lamp. Although those were surprisingly expensive in their day, they are quite economical now and widely available second hand provided one is willing to type a few searches. Before getting to much farther off topic, do consider picking up a Nicholas illuminator. Color temperature aside, the tight beam is well corrected and the arm most are fitted with is a great asset. Working with a Nicholas lamp and a compound microscope, we will quickly see why this method is far from ideal.

IMGP0005

Poorly set up.

In the above image we can see that a light source is set up sufficient to permit the specimen (here the engraved body of a pocket knife) to be brightly illuminated to the naked eye. There is enough working distance that no significant difficulty was involved in setting it up. A quick look through the eyepieces will immediately demonstrate that this set up is not only far from ideal, but entirely unsuitable. The light source is a painfully bright halogen bulb but the view from the oculars is quite dim, contrast is excessive, and there are visible color fringes even though the lowest powered objective (here a 30mm EF/3.5x achromat) is being employed. Some of these defects can be corrected even in this compromise set up.

IMGP0010

Ideal compromise set up.

In the above image is shown an exaggerated ideal arrangement of the available resources. If one takes the stage for a plain, and draws a vertical line along the visual optical axis the light from the illuminator should be arranged so as to be as at the most acute angle to the optical axis possible. This will go a long way to limiting the extremes of contrast and removing the color fringes. It will similarly  render the specimen as bright as possible under the present conditions.

It may not be immediately apparent but working distance is the limiting factor here. As soon as one moves beyond the 10mm or so that one is afforded by a standard 16mm (10x if you’re more comfortable with magnification than equivalent focus) objective the few working distance of a 4mm (43x) objective is far to short, even that of a rather less common 8mm (21x) objective will be much to short for all but the most intrepid of operators. Then, should illumination be secure one will be presented with a view of such poor quality that the effort is entirely wasted.

Next up… vertical illumination.

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