On the Rainbow
Latin Grammar: Interrogative Pronouns
Et perspectivi et physici est speculatio de iride. Sed ipsum "quid" physici est scire, "propter quid" vero perspectivi. Propter hoc Aristoteles in libro meteorologicorum non manifestavit "propter quid", quod est perspectivi, sed ipsum "quid", de iride, quod est physici, in brevem sermonem coarctavit. Ideoque in praesenti ipsum "propter quid", quod attinet ad perspectivum, pro modulo nostro et temporis opportunitate suscepimus explicandum.
Interrogative Pronouns
The interrogative pronoun quis, quid can be used to ask questions, either by itself or with prepositions as above. The ablative of the interrogative pronoun can also be combined with the suffix -cum to ask “with what (whom)?” as in quocum, quibuscum.