The work in the department of research in physiologic optics of the Dartmouth Medical School1 has demonstrated a number of facts in regard to the size and shape of ocular images:
The eyes are capable of perceiving a difference in size between the images in the two eyes of less than 0.25 per cent.1e
A difference in size not great enough to prevent fusion may cause all the well known symptoms of eyestrain. (This is possible with a difference of 0.50 per cent, according to the opinion of the investigators.1e)
It is possible to equalize the images in some cases by varying the base curve and the thickness of the lenses prescribed.
The method of these investigators either involves peripheral fusion, with fusion eliminated in the central area1 (see fig. 1), or depends on judgments in indirect vision as in their grid nonius tests, the first method
ALLEN DG. A TEST FOR ANISEIKONIABY THE USE OF CENTRAL FIXATION AND FUSION. Arch Ophthalmol. 1937;17(2):320–327. doi:10.1001/archopht.1937.00850020126008