Although the discovery of the horopter is universally credited to Franciscus Aguilonius (d. 1617) he did not describe the horopter in its modern sense. Ibn al-Haytham had described the concept of corresponding points and the horopter in its modern sense long before.
The horopter according to Aguilonius (d. 1617),
the locus in space within which both fused and diplopic images appear to lie
horopter in the modern sense,
the locus in space within which an object must lie to appear fused
Thus, says I. P. Howard, one cannot credit Aguilonius with the discovery of the horopter.
Aguilonius used his horopter instrument to plot the projected positions of disparate images within the plane of fixation rather than the locus of fused images. Aguilonius maintained that the horopter defined as the apparent location of disparate images is a frontal plane passing through the point of convergence. This is incorrect. It is now known that the horopter, defined as the locus of fused images, is approximately a circle passing through the point of convergence and the two eyes—the Vieth-Muller circle. Ibn al-Haytham had already proved in the eleventh century that the locus of fused images is not the frontal plane. Aguilonius had read Ibn al-Haytham and cited him four times. However, he did not refer to this proof or to Ibn al-Haytham's demonstrations on the limits of fusion and cyclopean vision.
In spite of the fact that Ibn al-Haytham had already proved that the locus of fused images does not lie in the frontal plane, the idea of a frontal plane horopter persisted until the early nineteenth century when Vieth used the same Euclidean theorems to prove that, he was apparently unaware of the contributions of Aguilonius or Ibn al-Haytham.