1. A method of determining when a subject's eye position is aligned with a reference axis in an external coordinate system, comprising: (a) determining positions of the sclera/cornea boundary (limbus) of the subject's eye in the external coordinate system, (b) from the positions of the sclera/cornea boundary determined in step (a) determining the center of the limbus of the subject's eye in the external-coordinate system, (c) determining the position of an image of a light beam reflected from a reflective surface associated with the subject's eye (d) determining that the subject's eye is aligned with the external-coordinate reference axis when the position of the reflection from step (c) is coincident with the center of the limbus from step (b).