Projection Micrography


Taking pictures of my work area makes me feel as if I should tidy up more often. -K

Projection microscopy can be a means to many ends. One might use it for group demonstrations, measurement, specimen comparison, even casual viewing if one is so inclined. Of course it’s looked at here for the purposes of micrography, to which it is particularly well suited. Micrography which is aided by projection of the image onto the drawing surface is considerably easier than many of the other methods to acquire. It is also among the more inexpensive methods though very much reliant upon the conditions of ones work area.

In the previous methods looked at one required only the usual set up and a skill at drawing with perhaps a graticule for assistance. In projection micrography one may be excused for initially thinking that a projection microscope is required, but that is just not the case; one can get by with little more than a microscope. Naturally there are bits of equipment than can simplify things, specialized accesories and specialized microscopes one can purchase, but if one is without funds to do so, or just eager to try projection micrography today with what is on hand there is no need to wait.

One will need a microscope which may be inclined so that the eyepiece is horizontal, a powerful illuminator, and paper and pencil. When gazing into the ocular one is presented with a magnified virtual image that is optimally viewed at the eyepoint of the ocular. However, when the eye is beyond the eyepoint of the ocular, or the ocular is removed, the virtual image can still be observed. An other image is produced however, a real image. This image can be thrown upon a screen or sheet of paper simply by placing it in the path of the rays which pass through the objective or objective and ocular. The size (not the magnification) of the image on the screen moderated by nothing more than the distance of the microscope from the screen.

With a traditional microscope one may incline the stand horizontally and using an external illuminator send light directly through the slide without making use of the mirror. If a wall (with a paper affixed to it) is sufficiently nearby, the room is sufficiently dark, and the illuminator sufficiently bright, one may simply focus the specimen by observing the image thrown upon the wall and trace it onto the paper. In some cases it will be helpful to use only objectives and oculars of the lowest power, or to employ only optics which have large object lenses that permit more light to pass though.

Drawing on a vertical surface is rather awkward and one would naturally prefer to have the image thrown onto a table. This is where various specialized bits come into play. Prisms and mirrors can be positioned so as to send the image from the microscope onto a table or wall regardless of the position of the body tube. The simplest sort of device is a mirror that may be positioned at forty-five degrees from the horizontal body tube, and constructed quite cheaply from a ladies compact. To size the image conveniently it may be necessary to place a book beneath the foot of the microscope. Below is an example of the set up which makes use of a mirror made for the purpose by the Bausch & Lomb company. The objective is a 32mm (160mm Tube Length) achromat.

Lange's is an indispensable resource in any lab!

Lange’s is an indispensable resource in any lab!

One might have seen projection oculars available and be tempted to believe that any poor results experienced are the fault of the optics. Before spending the money on specialty oculars (which have their uses) one might observe the images below. The first image is that from a regular 12.5x Huygenian by B&L, while the second is a 12.5x projection ocular by B&L. All else being the same, (except for the steadiness of my hand at the camera) one notices immediately that the first ocular produces a sharper, more tightly constrained image. Why then are projection oculars generally more expensive? Without getting into it, lets just say that they have their uses and under particular conditions to which they are suited they more than justify their expense. The demonstration here is to illustrate that one needn’t have a special set of optics to project an image for micrography.

A 12.5x Huygenian ocular

A 12.5x Huygenian ocular

A 12.5x projection ocular

A 12.5x projection ocular

The image below was made without any ocular at all. One can see that the letter “e” is oriented differently than in the images made with an ocular. If one cares to recall that there are no extra optics in the body of this particular microscope,only the objective and mirror being used to create the image, one can better understand something of optical principles. One should also note the uneven illumination of the field because this was example was not set up for either critical or Köhler illumination. Historically oculars were not used for much projection or photographic uses, the reason was largely related to the apparatus employed but it is worth mentioning that in most circumstances if an ocular is not used one can obtain a brighter image; by using a 40x objective alone rather than with a 4x objective and 10x ocular together for example.

To make it larger I need only have lowered the drawing surface

To make it larger I need only have lowered the drawing surface

Why not try projection micrography today! It can be a wonderful way to better understand the optical workings of ones microscope and produce micrographs at the same time. Below is a sketch of the letter “e” slide produced by this technique. As with the previous micrographs the sketch was made in under five minutes. Because of the method used more detail could be put into the image, which is also more precise than any of the micrographs produced previously.

More details and more accuracy, more involved set up too

More details and more accuracy, more involved set up too

If one wishes to ensure that the image is accurately projected be certain to use a reflector that is large enough, and near enough to the eyepiece (if one is used), to project the entire field of view. It is then a simple mater to measure perpendicular axes of the projected image to ensure it is circular.  As a final word on this method: the darker the work room the easier things will be.


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