A device for use in non-invasive monitoring of a human or animal subject's bodily functions in vivo, comprises: a first optical system for identifying the center (1) of a pupil (13) of an eye of the subject, said first system comprising a first light source (2) for directing light towards the eye, first receiving means for receiving light reflected from the iris (4) of the eye, and first processing means for determining the position of the center (1) of the pupil (13) from the light reflected (3) from the iris; a second optical system comprising a second light source (14) directing light to a focussing means (15) for focussing light in the plane of the pupil (13) and for directing the focussed light onto the retina (10) of the eye, a second receiving means for receiving light reflected (17) from the retina and back through the pupil (13), and second processing means for analyzing the light reflected from the retina (10); and alignment means for aligning the second system with the center (1) of the pupil (13) as determined by the first system.