52 Capt. Kater's experiments for determining the 
Hence, the value of one division of the micrometer appears 
t0 be 23T63 of an inch * 
In the course of these measurements, differences occurred 
for which I was at a loss to account ; but at length it ap- 
peared that they were to be attributed to remaining parallax ; 
for whatever care be taken in adjusting the microscope, it is 
scarcely possible to bring the image of the object precisely in 
the same plane with the threads, and the image will conse- 
quently be of various dimensions, according to its distance 
from this plane. Unless, therefore, the most minute atten- 
tion be paid to the adjustment for parallax, the error arising 
from this cause will be considerable ; and I may here remark 
that I believe the difficulty of bringing the image into the 
plane of the threads, to be the source of by far the most 
serious errors to which measurements by means of micro- 
scopes are liable. 
I had now to examine the equality of the threads of the 
micrometer screw. For this purpose, two fine lines were 
drawn near each other on a piece of brass, and the micro- 
meter being turned back as far as it would go, the distance 
of the lines was carefully measured ; and this was repeated, 
proceeding through the whole length of the screw, always 
advancing the micrometer one revolution previous to each 
successive measurement. The result of this severe test will 
best appear by giving the numbers themselves. 
