The Hess chart is used to assess diplopia and work out the defective muscle.
It shows the position of the non fixing eye in all positions of gaze when the other eye is fixating a target. Hence any under or over acting muscle can be identified.
Hess chart, left eye, showing superior rectus underaction.
Reproduced from Can myasthenia gravis be diagnosed with the ‘ice pack test’? A cautionary note, Andrew J Larner, Dafydd J Thomas, Postgrad Med J 2000;76:162-163. BMJ Publishing Group Ltd