You are observing a slide of green cells under the 4x objective. each cell covers approximately 1/8th of the width of your field of view. at least, that is what it seems is hard to get the sample cells to line up exactly and stop rotating. how large of a diameter do these cells have?

The microscope is an instrument used to see the smaller objects clearly. The microscope has different adjustment of lens  and can enlarge the image upto 100X.

The green cells has 4X objective and covers  1/8th of the width. The cell will approximately has the diameter of around 500-600 micrometers. This will accounts for its variability and the inaccuracies. The cells has size more than this diameter cannot be seen by the microscope.