in perimetry, the observer's eye is considered to be at the centre of an imaginary sphere. more precisely, the centre of the sphere is in the centre of the pupil of the observer's eye. the observer is looking at a point, the fixation point, on the interior of the sphere. the visual field can be considered to be all parts of the sphere for which the observer can see a particular test stimulus. if we consider this surface to be that on which an observer can see anything, then it is a section of the sphere somewhat larger than a hemisphere. in reality it is smaller than this, and irregular, because when the observer is looking straight ahead, his or her nose blocks vision of some possible parts of the surface. in perimetric testing, a section of the imaginary sphere is realized as a hemisphere in the centre of which is a fixation point. test stimuli can be displayed on the hemisphere.