What makes the letter e suitable observation under the microscope?

For teaching students about slides under a microscope, the printed lowercase "e" is used because it is identifiable even if only part of it is visible, and because it is asymmetrical both vertically and horizontally. You can see clearly how its image is changed under the microscope.

The compound refractive microscope inverts the view seen in the eyepiece, creating an upside-down image. When a higher magnification is used, only part of the "e" will fit in the field. This demonstrates that samples should be centered before changing to a higher power, or they may not be where they can be seen.