"Arm microscopes" is not a standard term in microscopy. It's possible you're referring to a specific type of microscope or a specific feature. To better understand your question, could you please provide more context or clarify what you mean by "arm microscopes"?

For example, are you referring to:

* Microscopes with an arm: Most compound microscopes have an arm that connects the base to the head, providing structural support.

* Microscopes used in surgery: Some microscopes are specifically designed for surgical procedures, which could be called "arm microscopes" in a specific context.

* Microscopes with a specific type of arm: Some microscopes may have a unique arm design for a specific purpose, like a flexible arm for positioning or a specialized arm for holding specialized attachments.

Once you provide more information, I can explain the function of the specific type of microscope you are interested in.