In the diffuse-porous woods, the demarcation between rings is not always so clear and in some cases is almost (if not entirely) invisible to the unaided eye. Conversely, when there is a clear demarcation there may not be a noticeable difference in structure within the growth ring.
What adjective describes how our eyes function without a tool like a microscope to help?
{'text': 'unaided', 'answer_start': 145}