66 
Dr. Young’s Lecture 
coincided. Neither could I observe any change in the images 
reflected from the other eye, where they could be viewed with 
greater convenience, as they did not interfere with the eye- 
glass. But, not being at that time aware of the perfect sympa- 
thy of the eyes, I thought it most certain to confine my obser- 
vation to the one with which I saw. I must remark that, by a 
little habit, I have acquired a very ready command over the 
accommodation of my eye, so as to be able to view an object 
with attention, without adjusting my eye to its distance. 
I also stretched two threads, a little inclined to each other, 
across a ring, and divided them by spots of ink into equal 
spaces. I then fixed the ring, applied my eye close behind it, 
and placed two candles in proper situations before me, and a 
third on one side, to illuminate the threads. Then, setting a 
small looking-glass, first at four inches distance, and next at 
two, I looked at the images reflected in it, and observed at 
what part of the threads they exactly reached across in each 
case ; and with the same result as before. 
I next fixed the cancellated micrometer at a proper distance, 
illuminated it strongly, and viewed it through a pin-hole, by 
which means it became distinct in every state of the eye; and, 
looking with the other eye into a small glass, I compared the 
image with the micrometer, in the manner already described. 
I then changed the focal distance of the eye, so that the lucid 
points appeared to spread into surfaces, from being too remote 
for perfect vision ; and I noted on the scale, the distance of their 
centres ; but that distance was invariable. 
Lastly, I drew a diagonal scale, with a diamond, on a looking- 
glass, (Plate III. Fig. 12.) and brought the images into con- 
tact with the lines of the scale. Then, since the image of the 
