Microscope Activities, 8: The Intermediate Image
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 8: The Intermediate Image
To become familiar with the location and nature of the intermediate image.
Jewelers’ screwdriver; sheet of blank white paper
- Using your 10X objective, focus on a high contrast specimen or stained tissue section.
- Adjust the illumination intensity to maximum.
- Using the finest jewelers’ screwdriver or small hex wrench, loosen the set screw that locks the eyepiece into the body tube; remove the eyepiece, and set it aside.
- Cut a sheet of blank white paper so as to form a strip that is about ½” wide and about 3” long. Fold about ½” of one end of the strip so as to form a “foot” that measures ½” x ½” (this is a makeshift projection screen).
- Push this mini projection screen down into the body tube for a distance of 1 cm. Figure 8-1 shows such a paper projection screen inserted 1 cm into the body tube; on the paper screen is seen the image of a stained wood section. It helps to darken the room somewhat.
The simple microscope consists of one stage of magnification, and is limited to relatively small enlargements. The compound microscope achieves much higher magnification by incorporating two (usually) or more stages of magnification. The Boreal/Motic microscope is compound; the objective, say 10X, forms the first stage of magnification (10X), with its image designed to be 1 cm down from the top of the body tube; this is the plane of the intermediate image. By placing your projection screen 1 cm down the tube, you are viewing the specimen magnified 10 times. The magnified view of the wood seen in Figure 8-1 is ten times larger than in real life, and is the same view you would get if you looked at the wood section through a 10X pocket magnifier.
Move the specimen slide forward and back, and from side to side, noting what effect this has on the image lying in the intermediate image plane. You will notice that this real image is inverted, relative to the object.
This intermediate image plane is also the plane of the eyepiece diaphragm, so that if you place a pointer or scale on the eyepiece diaphragm (Experiment 6), it will be superimposed right on top of the real image of your specimen. Look in the eyepiece again–from the bottom side–and notice where the shelf (diaphragm) and the pointer are located, relative to the shoulder where the eyepiece sits on the body tube – the distance will be 1 cm (10 mm).
Now, if you have a pocket magnifier, you can look at your intermediate image with it, so that you will be magnifying further the magnified image. The total magnification will be the product of the two magnifications. If your pocket magnifier is, say, 5X, and you have formed the intermediate image of your specimen with the 10X objective, the total magnification will be 5 x 10 = 50X. Your eyepiece is nothing more than a highly corrected magnifier that further magnifies the intermediate image by 10 times, for a total magnification of 10 x 10 = 100X.
It is convenient to have eyepieces of different magnifications available, so as to accommodate any specimen.
Some research-grade microscopes have a third stage of magnification. These auxiliary magnification systems, located somewhere between the objective and eyepiece, may be stepped (1.2X, 1.5X, 2.0X), or they may be continuous, i.e., zoom. One has to be careful in using these systems, however, not to exceed the maximum useful magnification (MUM), defined as 1000NA; one-thousand times the numerical aperture of the objective being used. With such a microscope, using the 100X objective, 25X eyepiece, and auxiliary magnifier set to 2.0, a final magnification of 100 x 25 x 2.0 = 5000X can be obtained.
Modern microscopes using digital capture devices for imaging are often designed to dispense with the eyepiece, and project the intermediate image directly onto the camera’s chip.
Perform the steps enumerated in the Procedure section. Try substituting a transparent metric rule or a stage micrometer for your specimen slide.