160 J. LeConte on Binocular Vision. 
There are few persons, I suppose, who have not — 
an unconquerable drowsiness while listening to a dull spea 
on a warm summer afternoon. Every one at such times a 
have observed that as the control over the ocular muscles is 
lost, the head of the speaker, whom he is vainly attempting to 
regard with attention, becomes double; the two heads ced 
ex eriments related below 
n accordance with the usual doctrine, I had long supposed 
that this don bine of images in sleepiness was due to optic con- 
vergence. On te nae it, however, I found I was mistaken. I 
tested es as follow 
easy, even in the state of drowsiness already mentioned, to per- 
form an experiment to test the position of the optic axes, As 
eleronymous images in this case prove optic divergence. Hor 
even with the point of sight a t infinite distance, i. e., the optic 
axes parallel, the doubling of an object at the distance of 30 or 
40 feet would be ‘almost imperceptible (the distance between 
the sonia. of the two images being only equal to the saps 
lar distance, or 24 inches), while in the experiments the images 
were widely separated, in some cases 10 to 16 feet, indicating 
therefore an optic divergence of 15° to 20°. 
am sure, in the course of 15 or 20 years I have performed 
this experiment many hundred times, and always with the 
leagues i pe University of cgass Carolina, who 
troubled a es milar drowsiness, to make the same experi 
ments. He did so, and his results were identical with my own. 
In every case, of course, the experiment rouses the mind an 
quickly re-unites the images, but not so quickly but that the 
