The Hess screen test is based on the haploscopic principle. It was popularized by Lyle, in particular for diagnosing possible paretic or paralytic conditions in patients with normal correspondence. 

Principles  Dissociation Foveal projection  Hering’s and Sherrington’s Laws of Innervation 

To perform this test, a black cloth 3 ft wide by 31‚ÅÑ2 ft long, marked out by a series of red lines subtending between them an angle of 5¬?, is used.¬† At the zero point of this coordinate system and at each of the points of intersection of the 15¬? and 30¬? lines with one another and with corresponding vertical and horizontal lines, there is a red dot.¬† These dots form an inner square of 8 dots and an outer square of 16 dots.¬†

An indicator is provided consisting of three short green cords knotted to form the letter Y.  The end of the vertical green cord is fastened to a movable black rod 50 cm long.  The ends of the other two cords are kept taut by black threads that pass through loops at upper corners of screen to small weights at corresponding upper corners of the screen. 

This arrangement enables the patient to move the indicator freely and smoothly over the whole surface of the screen in all directions. The patient wears red-green goggles and is seated 50 cm from the screen, preferably with his or her head fixed in a headrest.  The patient now sees the red dots with one eye (fixing eye) and the green cords with the other (charted eye). 

The patient is instructed to place the knot joining the three green cords over each of the red dots in turn.