55 
on the Mechanism of the Eye. 
Room was however still left for a repetition of the experi- 
ments; and I began with an apparatus nearly resembling that 
which Mr. Home has described. I had an excellent achromatic 
microscope, made by Mr. Ramsden for my friend Mr. John 
Ellis, of five inches focal length, magnifying about 20 times. 
To this I adapted a cancellated micrometer, in the focus of the 
eye not employed in looking through the microscope : it was 
a large card, divided by horizontal and vertical lines into for- 
tieths of an inch. When the image in the microscope was com- 
pared with this scale, care was taken to place the head so that 
the relative motion of the images on the micrometer, caused by 
the unsteadiness of the optic axis, should always be in the direc- 
tion of the horizontal lines, and that there could be no error, 
from this motion, in the dimensions of the image taken verti- 
cally. I placed two candles so as to exhibit images in a vertical 
position in the eye of Mr. Konig, who had the goodness to 
assist me ; and, having brought them into the field of the mi- 
croscope, where they occupied 35 of the small divisions, I 
desired him to fix his eye on objects at different distances in the 
same direction : but I could not perceive the least variation in the 
distance of the images. 
Finding a considerable difficulty in a proper adjustment of 
the microscope, and being able to depend on my naked eye in 
measuring distances, without an error of one 500th of an inch, I 
determined to make a similar experiment without any magnify- 
ing power. I constructed a divided eye-glass of two portions of 
a lens, so small, that they passed between two images reflected 
from my own eye ; and, looking in a glass, I brought the appa- 
rent places of the images to coincide, and then made the 
change requisite for viewing nearer objects : but the images still 
