Expert: Walter Hintz - 10/10/2009

Question
I have been looking over my biology book for days and cant figure this out. How do I estimate the size of an object viewed through a microscope when given the diameter of the field of view? Is there a specific formula, or is it easier than that?

Answer
Hi Jennelle
Lets assume that the  diameter of the field measures 2mm at 40x (measured with 4x lens in place. You can find this out by placing a plastic mm ruler under the four power lens.) Now if you look at a protozoan like a paramecium under the scope. Line up one end of the organism at the edge of the field and if it reaches 3/4 of the way across the field it measures 1.5 mm. Now comes the tricky part. Lets move the 10x lens in position ofr a smaller organism. Since we know the diameter of the field at 4x is 2mm then we can determne the diameter at 100x.
Diameter of the field at 40 x = 2mm                2000 microns
Diameter of the field at 100x = 4/10 x 2= 0.8 mm    800 microns
 Diameter of the field at 400 = 1/4 x 0.8 = 0.2 mm  200 microns