CELESTIALMAGNITUDES 111across thecenterof the fieldof view, and pictured
uponthe heavens. These are withinthe controlof
theobserver. He canincreaseordecreasetheir dis-
tanceatpleasure,andsorevolvethemastobringthem
into any position, every motion being accurately
measuredbyproperlydividedscales.
"Suppose, then,it is desired to takethe distance
andposition ofthe stars forminga pair. Thetele-
scopeisdirectedtothem,andtheyarebroughttothe
centerofthefieldofview. Theclockworkis setin
action;ittakes uptheponderousinstrument, weigh-
ingmorethan2,500 pounds,and with themost as-
tonishing accuracy it bears it onward, keeping its
mighty eye fixed on the object under examination.
Theobserveristhusleftwithbothhandsfreetomake
hismeasures. Hefirstrevolveshismicrometerspider's
linesrounduntilone ofthemshallexactlypassfrom
centerto center of the two stars. This position is
noted,and from itis deductedtheangle framedby
thisline withthe meridian. He then revolvesthem
aquarterofthecircumference,andtheyareperpen-
dicularto their formerposition. Henow separates
thewires untilthe oneshall exactlybisectone star,
whiletheotherwirepassesthroughthecenterofthe
secondstar,readingthisdistanceontheproperscale.
Hehasfixed, inthesetwo observations,theposition
and distance of the two components of the double
set. Suchistheprecisionattainedinthiswork,that
themostminutemotions cannotescapedetection. If
thestarsseparatefromeachotheratsoslow arate
thatamillionofyearswouldbe requiredtoperform