L&S, http://www.perseus.tufts.edu/hopper/text?doc=Perseus%3Atext%3A1999.04.0059%3Aentry%3Din1 wrote:7. Sometimes with esse, habere, etc., in is followed by the acc. (constr. pregn.), to indicate a direction, aim, purpose, etc. (but v. Madvig. Gram. § 230, obs. 2, note, who regards these accusatives as originating in errors of pronunciation); so, esse in potestatem alicujus, to come into and remain in one's power: esse in mentem alicui, to come into and be in one's mind: esse in conspectum, to appear to and be in sight: esse in usum, to come into use, be used, etc.