Why Huygens eyepiece cannot be used with cross wires?

Answer 1

When measurement of final image is required then the cross wire should be placed between the field lens and eye lens. But the cross-wire is viewed through the eye lens only, the distant is viewed by ray refracted through the lens.Due to this reason,relative lengths of the cross-wire and the image are disproportionate. Hence cross-wire cannot be used in Huygen's eyepiece and this is a disadvantge.

Answer 2

Huygens eyepiece uses two lenses that can cause distortion in the cross wires when passing through them, making it difficult to accurately view objects. The design of the eyepiece can also cause the wires to appear out of focus or misaligned, affecting the observer's ability to make precise measurements or observations.