Because each haploscope arm, mirror, and CRT rotates about the center of rotation of the appropriate eye, we can change the headcentric azimuth of the stimulus without altering the retinal images.
Spatial calibration is accomplished as follows.
The loom is mounted 1 cm in front of the CRT to be calibrated, and is viewed in the mirror from the standard viewing position. An observer positions a dot to be coincident with specified intersections in the grid. When the dot is aligned with an intersection, its coordinates are recorded. The loom is removed during experiments. Its former location defines a virtual plane onto which stimuli were projected.