1. Compound Light Microscope

2. TOTAL MAGNIFICATION
Power of the eyepiece (10X) multiplied by objective lenses determines total magnification.

3. Magnification Objective Ocular Eyepiece Total Magnification Low Power
4 x
10 x
40 x
Medium Power
100 x
High Power
400 x

4. Field of View
Field of View (FV) is the illuminated circle that you see when looking through the ocular eyepiece.
If we know the diameter of the FV then we can estimate the size of our microorganisms.

5. Field of View
With our microscopes the diameter of the FV under low power is 4 mm
FV is measured in micrometers or microns.
1 mm = 1000 microns
Therefore, our FV under low power is 4000 microns

6. Using the Field of View to Estimate Microscopic Measurements
If an organism takes up ½ of the FV under low power, it must be about 2000 microns in length

7. Making cheek cell and onion cells slides
Scrape the inside of your cheek with a toothpick
Rub toothpick onto the slide and add a drop of methylene blue
Place a cover slip onto the slide. Observe
Onion
Peel off inner epidermis of onion using forceps
Place on slide
Put a drop of iodine and a cover slip onto your slide
Observe

8. How does the FV change as Magnification goes Up??
As magnification goes up, the size of the FV gets smaller.
If magnification increases 2x, the FV is divided by 2x, or 2x smaller.
If we switch from low power (4X) to medium power (10X), the increase in magnification is 2.5 times (10X/4X).
The FV under medium power will be 1600 microns (4000/2.5)

9. Calculating the Diameter of the Field of View
Step 1 Calculate the Increase in Magnification New Objective
Old Objective
Step 2 Divide the old F of V by the increase in magnification calculated in Step 1
Old F of V (Microns)
Increase in Mag

10. To Calculate the Changing FV
Low – 5x; Med – 10x; High – 50x
Low FV = 5mm (1000um x 5mm= 5000um)
Step 1 – Calculate the increase in Magnification
Ex – 10x/5x = 2
Step 2 – Calculate the reduction of the FV
5000um/2 = 2500um