Microscope Activities, 6: The Eyepiece (Ocular)
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 6: The Eyepiece (Ocular)
To understand the construction, characteristics, and use of the eyepiece.
Metric rule; pocket magnifier (or another eyepiece); jewelers’ screwdriver (the finest size in the usual set of six sizes), or hex wrench
- Using the 10X objective, focus on any prepared specimen (rotate the rheostat to obtain maximum brightness).
- In a semi-darkened room, place the metric ruler vertically in contact with the eyepiece, in such a way as to bisect the emerging light rays (Figure 6-1), and read the distance to the exit pupil or eyepoint.
- Examine the eyepoint, using a pocket magnifier (Figure 6-2) or spare eyepiece held inverted.
- Using the finest jewelers’ screwdriver or small hex wrench, loosen the tiny screw on the underside of the eyepiece tube that prevents the eyepiece from being removed, until you can remove the eyepiece. Disassemble the eyepiece by unscrewing the upper optical portion from the lower half containing the pointer (Figure 6-3).
Figure 6-1 shows that the light rays from the eyepiece converge at approximately 10 mm above the top of the eyepiece, before again spreading out. This 10 mm distance is known as the eye relief. In the past, the eye relief tended to be quite short, 1-5 mm, so that the microscope user had to have their eye very, very close to the top lens in order to prevent vignetting. Later developments saw the introduction of high eyepoint eyepieces, 10 mm or more. This allowed even users who had to wear corrective eyeglasses to use the eyepiece without vignetting. Users who are either nearsighted or farsighted may use the microscope without their eyeglasses; users who have astigmatism need to keep their eyeglasses on (although there is a trick: lining up your sample in the one direction where you are free of astigmatism; this requires asking your optometrist or ophthalmologist for the degree of orientation).
The eyepiece that comes with the Boreal/Motic microscope bears the legend: WF10X-18 mm. The WF stands for wide field, and the diameter of the intermediate image, which lies in the plane of the eyepiece diaphragm, where the pointer is located, is given as 18 mm. The magnification is given as 10X.
Your eyepiece does not say that it is high eyepoint, but, at 10 mm, it is. High eyepoint of an eyepiece can also be designated graphically with a tiny pair of eyeglasses, or by the letters H.E.P.
10X is a typical eyepiece magnification. The lowest magnification of eyepieces for visual use can be 5X, 6.3X, 7X, and 8X. Lower magnification eyepieces, 1X, 2.3X, 3.2X, 4X, etc., are used for projection only.
Eyepieces of magnification higher than 10X include 12.5X, 16X, 20X and 25X. The most commonly used eyepiece magnifications are 10X and 12.5X.
Hint to teachers: since the Boreal/Motic microscope is a monocular instrument, this is a good opportunity to notice which eye each student uses to look into the microscope; they will unconsciously use their dominant eye. This will become important when they use binocular heads, because any reticle or graticule, such as crosshairs, measuring scale, Whipple disc, etc. being used in the eyepiece should be placed on the dominant eye side.
The eyepoint that we have shown to be 1 cm above the top of the eyepiece is actually an image of the objective back focal plane. The objective back focal plane, which lies at different levels within each different objective, is where we also find the image of the filament when using an incandescent bulb as light source, and the image of the aperture diaphragm which is located at the lower end of your substage condenser (we will look at this in another experiment). For the moment, what we may do is at least look at our eyepiece eyepoint, using a pocket magnifier, or by using another eyepiece in inverted position.
Figure 6-2 illustrates the eyepoint being examined with a pocket lens; note that the eye is brought close to the magnifier, and the pinky finger is used to steady the hold. What you will see is a disc of light. The reason you do not see a filament is because your light source is an LED and does not have a tungsten coil, and, more importantly, because you have two ground glass diffusers in the light path beneath the condenser. You should, however, be able to see the leaves of the aperture diaphragm; if you do not, reach down with your hand and move the lever that controls the aperture diaphragm (there will be an experiment on this later).
Instead of examining the eyepoint with a magnifier as just described, you may simply remove the eyepiece, and look down the body tube! The image will be smaller, but the same aperture diaphragm image. Alternatively, a pinhole eyecap can be used—or just punch a hole in a piece of paper with your dissecting needle, place the hole centered on top of your body tube, place your eye very close to the paper, and you will see a sharp image of the objective back focal plane!
The objective back focal plane is so important in unraveling the mysteries of crystals, that optical crystallographers have an extra lens system built into their body tube: the Bertrand lens. This lens can be flipped in and out of the light path to give a magnified view of the objective back focal plane (the Fourier plane).
Now, remove the eyepiece and unscrew it (Figure 6-3). Notice that the upper part contains all of the lenses; the lower part has no lenses, but it does have a shelf that supports the pointer. This shelf is the eyepiece diaphragm; it is this diaphragm which defines the round shape of your field of view—the 18 mm designation, printed in white on your eyepiece, is the actual diameter of the opening; here is also where you would place any measuring graticule, crosshairs, etc. which are normally engraved on glass, or consist of photo reductions of scales, etc. Ideally, any eyepiece with a pointer, reticle, or graticule should be supplied with a focusing eyelens; your eyelens is fixed, so that not every user will see the built-in pointer in sharp focus.
The set-screw preventing the eyepiece from being removed is intended to make the eyepiece student proof, and to keep the eyepiece from falling out if the instrument is inverted. It is far better, and, ultimately, in the students’ interests, to teach good techniques from the outset; therefore, loosen or remove the set-screw, and teach responsible handling and use of this precious instrument.
Another advantage of making the eyepiece removable is that an unlimited number of other eyepieces can be substituted for the supplied 10X pointer eyepiece. For example, the rack of eyepieces in Figure 6-3 contains a lower magnification (6.3X) eyepiece at the far left, followed by a pointer eyepiece—the little lever at the top of the eyepiece allows for the movement of the pointer. The third eyepiece in the rack is a higher magnification eyepiece with high eyepoint. The eyepiece on the far right has a focusing eyelens. Any one of hundreds of different graticules may be placed on the diaphragm inside, and may be brought into focus by most people (users with extreme correction—up to +6 diopters—will need to leave their corrective lenses on).
Loosen the tiny screw that retains the eyepiece using the smallest screwdriver in a set of jewelers’ screwdrivers, or a small hex wrench, depending on whether a slotted screw or hex-head set screw is present. Remove the eyepiece from the tube and unscrew the two parts. Note that the lenses are all in the upper part, and the diaphragm with pointer is below the lens system. This is known as a positive eyepiece, or one of Ramsden design. Eyepieces in which the diaphragm lies between the lens systems are known as negative eyepieces, or of Huygenian design.
See also Experiment 10 for another demonstration of the eyepoint.