What is used as a measuring scale in the eyepiece of a telescope or microscope?

The reticle is a grid or system of markings that is superimposed on the field of view in the eyepiece of a telescope or microscope. It serves as a measuring scale, allowing users to make precise measurements of the objects they are observing. The reticle can aid in determining the size and distance of objects when used in conjunction with the optical instrument. It provides a reference point that enhances accuracy, especially in applications like surveying or microscopy where precision is critical.

The focus ring, while essential for adjusting visual clarity, does not serve a measuring function. Eyepiece filters, which can enhance or modify the light entering the eyepiece, do not inherently provide a scale for measurement. The optical lens, an integral part of the optical system, helps in magnifying the image but does not function as a measuring scale itself. Thus, the reticle is specifically designed for measurement purposes, making it the correct answer.