An outer part for a device which is attachable thereto as a housing and/or an attachment part, a first reflective surface and a second reflective surface being formed on the outer part so that at least one signal emitted by an optical and/or acoustic source is directly or indirectly deflectable onto at least one detector surface of an optical and/or acoustic detector, an optical path being configured as a cavern or continuous recess in the outer part or as a depression of a boundary surface of the outer part, the optical path having at least one opening via which at least one substance is transferable into the optical path, and the at least one signal being deflectable into the optical path to the second reflective surface which is formed at a second end of the optical path with the first reflective surface at a first end of the optical path.