If you observe a cell without a nucleus under the microscope, you are likely observing a prokaryotic cell.

Here's why:

* Prokaryotic cells are simple, single-celled organisms (like bacteria and archaea) that lack a membrane-bound nucleus and other membrane-bound organelles. Their genetic material (DNA) is located in a region called the nucleoid, but it's not enclosed within a membrane.

* Eukaryotic cells, on the other hand, are more complex cells found in plants, animals, fungi, and protists. They have a true nucleus that contains their DNA, as well as other membrane-bound organelles.

So, the absence of a nucleus is a key characteristic that distinguishes prokaryotic cells from eukaryotic cells.