84 J. LeConte—Phenomena of Binocular Vision. 
concerning this law had seemed to me inconsistent with each 
I therefore read again carefully and thoughtfully Helm- 
holtz’s great work on Physiological Optics, the acknowledged 
standard on this subject. I read and re-read several times his 
chapter on the laws of ocular motion, and pondered upon his 
results. I repeated all his experiments, and made many more 
of my own. But so difficult and delusive are experiments of 
this kind, so beset on every side with sources of fallacy, that 
the more I experimented and pondered, the more I became 
bewildered. But now at last the whole subject has become 
not so much the delusiveness of the phenomena, but the too 
ardent desire to verify the results of ethers, rather than to 
determine the law for myself. I have been driven almost 
against my will to the conclusion that there are some strange 
and apparently inadvertent mistakes in Helmholtz’s interpre- 
tation of Listing’s law, and that this law governs the motions 
of the eyes only when they move parallel to each other, but 
cannot in any way account for the torsions of the eyes in 
being in fact the outward manifestation of images as it were 
burned into the retina—they must of necessity follow with the 
greatest exactness all the motions of the eye. There is no other 
mode of detecting torsions of the eyes, in parallel motion. All 
my experiments, therefore, were made with these images. 
wment 1.—I darken the experimental room by closin 
the shutters, but allow the light to enter through a narrow 
vertical slit between two shutters. I now gaze steadily with 
hea t on the vertical slit for a minute or so. On turn- 
ing to the blank white wall I see distinctly a colored verti- 
cal spectral image of the slit. I arrange my head if neces- 
sary, so that the image is perfectly vertical. If I now turn 
