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Unlike van Leeuwenhoek’s single-lens microscope, we now combine the magnifying power of multiple lenses in what is called a compound microscope.

Though van Leeuwenhoek’s apparatus was simple, the magnifying power of his lenses and his curiosity enabled him to perform great scientific observations on the microscopic world. He was ridiculed for fabricating his observations of protists at first. Ever the scientist, van Leeuwenhoek examined samples of his own diarrhea to discover Giardia intestinalis. While he did not make the connection of the causative nature of this microorganism, he described the details of the way this organism could propel itself through the medium in great detail.

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In an even more extreme close-up (higher magnification), we would have difficulty focusing on both the eyes and beak since there is depth and distance between those features.

Magnification is the process of enlarging the appearance of an object. We calculate the magnification of an object by indicating the fold change in size. So if something appears to be double the size of the real item, then it is obviously magnified 2X. Because there is a magnification by the eye-piece (ocular lens), as well as the objective lenses, our final magnification of an item is the product of those two lenses.

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In a microscope, we ordinarily observe things within a circular space (or field) as defined by the lenses. We refer to this observable area as the field of view (FOV). Understanding the size of the FOV is important because actual sizes of the object can be calculated using the Magnification of the lenses.

In image 1, we can see a model of DNA on a table with a water bottle and a large area of the room. Image 2 displays less of the room in the background but the DNA model is larger in appearance because the magnification is greater. In image 3, we no longer see evidence of a door and the DNA model is much larger than before. In image 4, we no longer see the table the model and water bottle rest upon. While the last image is largest, we see less of the surrounding objects. We have higher magnification at the cost of the field of view. FOV is inversely related to the magnification level.

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This page titled 1.2: Microscopy is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Bio-OER.

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In 1665, Robert Hooke published Micrographia, a book that illustrated highly magnified items that included insects and plants. This book spurred on interest in the sciences to examine the microscopic world using lenses but is also notable for Hooke’s observations of cork where he used the word “cell” in a biological sense for the first time.

We know that the water bottle is behind the DNA molecule. Under the microscope, the threads of differing color are also stacked on top of each other. We recognize that they are on different planes because they are three dimensional. Each thread has depth and does not occupy the same exact space. If we focus on the print of the water bottle on the image above, we would no longer see the lettering on the DNA molecule sharply. We refer to this concept as Depth of Field (DOF). Under the microscope, at low magnification, we can make out fewer finer details. However, most items appear on the same plane in this case and or comparably sharp. But as we increase the magnification and see finer details, the distances between the various planes in view become more apparent. We can see a similar phenomenon at low magnification of the DNA model. At the low magnification, we may not be able to read the print on the water bottle, but the bottle and DNA molecule are of a similar distance from our view that the small difference in apparent depth is not as noticeable. We can still draw on other visual cues to know that the bottle is behind the model, but the sharpness of both items are equivalent.

1. Examine the slide of colored threads under scanning power so the cross-point of the threads is at the center of the field.

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The lowest magnification objective lens (usually 4X or 5X) is referred to as a scanning lens. There is also usually a low power lens at 10X and a higher magnification lens at 40X. There may be a higher magnification lens at 100X but these usually require oil to function properly and are often reserved for microbiology labs.

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We notice that when we observe 3 overlapping threads of different color under a microscope, we can focus on one thread at a time. Similarly, when we zoom in a great deal on the DNA model below, we notice that the print on the water bottle is not sharp.

The Dutch tradesman Antonie van Leeuwenhoek used high power magnifying lenses to examine the parts of insects and to examine the quality of fabric in his drapery business. He began to experiment with pulling glass to generate lenses and developed a simple microscope to observe samples. Using a simple single lens with a specimen mounted on a point, he was able to identify the first microscopic “animalcules” (little animals) that will be later known as protozoa (original animals).

In our lab, we look at some pond water. What do we see? Why is this significant? How does the microscope help us study these items? What is the utility of the concepts of magnification, FOV, and DOF when we use microscopes to study biological samples?

Highest Magnification with shallow depth of field. Notice how the label on the water bottle is blurry while the lettering on the DNA model is sharp.