One part of our exercise involves the direct measurement of the diameter of the field of view using a clear ruler at 40x and 100x. Because the working distance of the 40x objective is shorter than the thickness of that ruler, the diameter at 400x cannot be directly measured. To determine this value an equation is given in the manual that requires the use of the measured value of the diameter at one of the other magnifications.
While memorizing this equation is not important, understanding how it is derived is easy and will reinforce your understanding of the relationship between the diameter of the field of view and the total magnification.
The relationship between the diameter of the field of view and the magnification is inversely proportional: mathmatically it can be expressed as
Now by simply multiplying both side of the equation by the diameter of magnification "A" we arrive at the equation given in your lab manual