C. A. Young—Diameters of Mars. 207 
cope doubles distinctly, and on fine nights easily, the little com- 
panion of a’ Capriconi, nor has it yet failed upon any of the 
objects usually considered tests for twelve-inch instruments. 
hile perfectly aware that measures by a filar micrometer 
are subject to a considerable constant error, the measure of an 
illuminated disc being always excessive, I have thought they 
might safely be used in determining a difference of diameter in 
different directions. The wires were dark upon an illumina- 
ted field. The magnifying power throughout the observations 
was 398. The value of one revolution of the micrometer 
screw at 50° F., as determined by about a hundred star tran- 
sits, is 17’’-860+-0-003. The screw appears to be a very per- 
fect one without sensible errors of periodicity or run. 
Marth’s ephemeris, in the Monthly Notices for June, 1879, 
Was used in setting the position circle and in computing the 
minute phase-corrections. Each measure of a double diame- 
ter consisted of twenty settings of the micrometer wire, five 
with the movable wire on one side of the fixed wire, ten with 
it upon the other side, and finally five more in the first posi- 
ion. The correction for thickness of the wires was deter- 
mined by a full set of readings at each of the three different 
fixed wires, once at least in each evening’s work. 
fter each measurement of either diameter, the wires were 
turned 90° by the position circle, and the other diameter was 
measured. In some cases, however, after finishing a set of 
measure of the polar diameter being matched with an equato- 
Lal immediately preceding or following. The only exceptions 
micrometer readings, exclusive of those for determining the 
thickness of wires, was 1140. ; : 
Had not cloudy weather prevented, the series would have 
been much more extensive. The nights of November 11th 
and 18th were entirely overcast; on November 12th only a 
