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1. MEASURING OBJECTS UNDER THE MICROSCOPE
Imagine a photograph of you when you were a child, 1.2m tall. The family dog is there right beside you and in the photo it is about half as high as you are. How high is the dog?
Yes, it is that simple.
You have an object under the microscope. The FIELD of view is a circle of light. The object is smaller than that. You have already roughly measured the FIELD and for that lens on your microscope (microscopes vary) let's say it is about 4.5 mm, or 4 500 micrometers (a.k.a. microns).
So,  you already know the object's length is smaller than 4 400µm. If your calculation gives something bigger than that, there is an error.
Next,  to calculate the object's length, first identify what you mean by the length.
 Once you have identified the length, figure out what portion of the FIELD it covers. It may help you to imagine dividing the field into half, then dividing a half into two quarters, then dividing a quarter into two eighths, until you get to the size of the organism. That fraction times the FIELD is the approximate length of the organism. Here, the organism's length is about 1/4 of a FIELD, or 0.25 X 4500µm = 1125µm.
 To get the WIDTH: either do all of that again for the width, or eyeball the animal and estimate the width as a fraction of the length. For our organism we might say width = about 1/3 of length. Having calculated length already, L = 3W, so 1125µm=3W, W=375µm.
It does help to structure the relationships as simple formulas, and certainly check that L is not greater than FIELD (unless it really is), W not greater than L, etc. (Don't lose marks for mistakes that are that preventable.)
You need to know  the actual size of the object you drew, and  the actual size of the drawing (or photo, etc.).
Magnification of the drawing is: Drawing_size/Object_size.
Magnification power of the microscope is the product of the magnifications of all the lenses in the system, e.g. 10x ocular lens and 40x objective lens gives microscope magnification 10 x 40 = 400times.
Magnification of the drawing is NOT the same as the magnification power of the microscope. If you need to mention magnification of the microscope, write "as viewed under microscope at 400X" or some such.
2a. Scale bars
It is often desirable to put a scale bar on a drawing. The scale bar is like a little ruler that applies to that drawing. You see scale bars on maps ... a little bar that says 100km for example.
Suppose your drawing is of an organism that is actually 1125µm L and 375µm wide (figured out as above). Your drawing of it is 6.2cm L and 2.1 cm W. (That big measuring bar is just to show you how to figure things out, it might not meet the requirements for a drawing in any particular course (the requirements are told to you in lab, we can only help you figure it out).
On your drawing, therefore, 1125µm is represented by 6.2cm=62mm=62 000µm. The MAGNIFICATION OF THE DRAWING is therefore 62 000/1 125 = about 55 times (you can write something like "drawn at 55x actual size").
Suppose you want a scale bar to represent 500µm. Get the length of the scale bar from the proportions of your drawing. Either measure [55 X 500µm], or 62cm X (500/1125)*, and draw your scale bar.
*[scalebarsizeonpaper = drawing_size X scale_represented / actual_size]
Check the assignment instructions to see what you are supposed to draw, label, etc., and how you are supposed to draw it. This page does NOT replace the instructions given you for the course.