Which factor does NOT affect the measurement of a cell's size under a microscope?

The measurement of a cell's size under a microscope is influenced by various factors, including magnification levels, the type of staining used, and the type of microscope employed. Magnification directly impacts how large or small the image of the cell appears, allowing for better visualization of cells at varying sizes. The type of staining can affect contrast and visibility, which influences how clearly the cell's size can be observed. Additionally, different types of microscopes have varying capabilities in terms of resolution and magnification which also play a crucial role in accurately measuring cell size.

Environmental temperature does not influence the measurement of a cell's size directly. While temperature may affect the physiological state of cells and could lead to changes in cell structure or behavior, it does not alter the fundamental ability to measure a cell's size through microscopy. The other factors relate closely to the optical properties and methods of visualization rather than external physical conditions. Therefore, environmental temperature is the correct choice as it does not have a direct effect on the measurement of a cell's size.