apparatus is first/second as a past participle. I suppose it could be used as a substantive, but there is also a fourth declension noun with the same lemma mean

apparatus is first/second as a past participle. I suppose it could be used as a substantive, but there is also a fourth declension noun with the same lemma meaning implements, tools, etc.: https://logeion.uchicago.edu/apparatus https://logeion.uchicago.edu/apparatus