112 L. M. Rutherfurd—Measurement of Angles. 
Art. XV.—A Glass Circle for the measurement of Angles; by 
Lewis M. RUTHERFURD. 
At the summer meeting of the National Academy of Sciences, 
in the year 1866, I described the micrometer which I had con- 
structed for the measurement of astronomical photographs. It 
was capable of measuring angles of position, and also distances 
in directions at right angles to each other. These last measure: 
ments were made by aid of screws arranged after the manner of 
those of an ordinary slide rest: these screws were constructed 
with great care and I had good reason to be satisfied with the 
smallness of their errors. 
At the Spring meeting of the Academy, for the year 1870, 1 
explained that I had been obliged to give up the idea of using 
screws on account of the rapid changes in their errors caused 
by friction and consequent wear, and I then stated that I in- 
tended to discard the screw and the compressed slide, and sub- 
stitute for them a divided glass scale, to be read by a microme- 
ter microscope, and a gravity slide with one V and one flat slide. 
This intention I earried out during the year, the new form be- 
ing first used about the month of March, 1871. It has been 
constantly used since that time, and continues to give great 
satisfaction. The success of this divided glass scale confirmed 
n a determination of long standing to try the experiment 
of substituting a glass circle for one of metal in some instru- 
me the most promising success. I had in my possession 
pe. 
was able to furnish to Mr. Stackpole a well tried diamond 
which has made lines of the greatest delicacy, being much finer 
