Microscope Activities, 26: The Aperture Wheel
In the past, Hooke College of Applied Sciences offered a microscopy workshop for middle school and high school science teachers. We thought that these basic microscope techniques would be of interest not only for science teachers, but also for homeschoolers and amateur microscopists. The activities were originally designed for a Boreal/Motic monocular microscope, but the Discussion and Task sections are transferable to most microscopes. You may complete these 36 activities in consecutive order as presented in the original classroom workshop, or skip around to those you find interesting or helpful. We hope you will find these online microscope activities valuable.
EXPERIMENT 26: The Aperture Wheel
To learn the function and use of the aperture wheel, when a substage condenser assembly with aperture iris diaphragm (See Experiment 7) is absent.
Look beneath the stage of your microscope to determine whether your microscope is equipped with a substage condenser with aperture iris diaphragm (See Experiment 7, Figure 7-4), or whether it is equipped with an aperture wheel—an aperture wheel is a rotating disc of metal or plastic with different size holes in it.
In Figure 26-1, the substage mirror and filter carrier of this early 20th Century Bausch & Lomb microscope have both been swung out of the way to reveal the circular piece of metal that, when rotated, introduces different-size holes into the light path; this is the aperture wheel.
Figure 26-2 is a close-up view of the stage of an American Optical Company microscope. Protruding from just beneath the front of the stage is the aperture wheel—in this case a knurled plastic disc, here indicating with the arbitrary marking “IIII” which hole diameter is in the light path.
In Figure 26-3 the microscope has been tilted backward so as to show where the aperture wheel with its different diameter openings is located.
If we remove the screw that holds the aperture wheel in place, and about which it rotates, we can remove the aperture wheel to see how it is constructed (Figure 26-4). We see that in this example, there are five holes of different diameter, and a set of five different designations—I, II, III, IIII, IIIII set opposite each hole. When “I” is seen at the front from above the stage, the largest hole is in the light path; when “II” is seen below the front of the stage, the next smaller hole is in the light path….and so on, until the “IIIII” indicates that the smallest hole is in the light path. Look at Figure 26-2 again; the “IIII” indicates that the next-to-the-smallest opening is in the light path.
Now that we know what an aperture wheel looks like, and where it is located, we need to learn how, when, and why to use it. Start by reviewing the Discussion section of Experiment 7, which describes the use of the variable aperture iris diaphragm. The iris diaphragm that constitutes the variable aperture iris diaphragm is relatively more expensive to manufacture and assemble than a simple disc of metal or plastic with 4-5 holes drilled or molded into it; that is why aperture wheels tend to be found on less expensive microscopes.
The function of both the aperture diaphragm and the aperture wheel is, however, the same; namely, to control the resolution, depth of field, and contrast of a microscopic specimen. The non-mathematical principle is simple: The more open you adjust the iris diaphragm—or the larger the hole of the aperture wheel you use—the greater will be the resolution (resolution: ability to discriminate fine spacing between adjacent structures), but the lesser will be the depth of field (ability to have higher and lower levels in focus at the same time when focused on a specific plane), and the lesser will be the contrast (relief; difference between lights and darks). Conversely, the more you close the aperture diaphragm—or the smaller the hole of the aperture wheel you use—the less will be the resolution, but the greater the depth of field, and the greater the contrast within the specimen. In practice, for the vast majority of specimens, neither wide open (largest opening) nor fully closed (smallest opening) is optimum.
For most specimens, the best starting position of the aperture or opening is one in which the iris or hole just comes in ~15% or so into the objective back focal plane view. The objective back focal plane may be viewed by removing the eyepiece and looking down the body tube, or, if the eyepiece is locked and non-removable, by viewing the eyepoint with a pocket magnifier (See Figure 6-2). Each objective used will have a different setting of the aperture diaphragm or wheel, because of the different numerical aperture that each objective possesses. The very least that should be done is to get into the habit of trying each aperture wheel setting with each objective with each specimen to select that opening that yields the best balance of resolution, depth of field, and contrast; the worst possible practice is to avoid or ignore the aperture diaphragm or aperture wheel setting.
Select a variety of prepared specimen slides and live specimens, and view them with each objective available, and at each aperture wheel setting. Make notes as to which combination is best for each objective, aperture wheel setting, and specimen type.
Note 1: The black plastic aperture wheel of the AO microscope (Figure 26-4) has the I, ll, III, IIII, IIIII designations molded right into the black plastic, and is installed that way. What we did here was to fill in the designations with White Correction Fluid to increase the visibility (Figure 26-2) and make them more difficult to ignore!
Note 2: Referring to Figure 26-3, once the aperture wheel was removed, we get a chance to see that this particular microscope model has a fixed, non-focusable substage condenser lens press-fitted into the stage opening (Figure 26-5). The threaded hole at the top is where the center of the aperture wheel is screwed in, and the bent spring-metal piece at the bottom is what engages the detent in the margin of the aperture wheel to form the click-stop; compare to Figure 26-3.