An introduction to astronomy: designed as a text-book for the use of students in college. By Denison Olmsted ...

58 tilfTE E ARz.l1'29.::, he sc.sda'.int is one of thie molst usefiul istriuments, both
to the astronoml er and the t navigator, and will therefore enrtit
partioular: attention::.;Inl figure t19,.1' rand i[l are ttwo smtall
mirrors, mald Ti a smlall telescope. I ) represents a movabhlet
arWl, or radius, which carries an index at tI):: Thle radiut tlurns
on a pivot at 1,;:i and the indoex mov es on a gradluatedt are ET.1
fig, 19.
I is called thle ha/izex 01fis and tlIf the I./rvin. (lGa-ss. %The1
under par't only of the holizoli ghass is coatted mrith quiebksilver,
thole tupter' p)ar't beirg left tn ncpualr elt; u so tflnt whilec: onle object
is-seten thromghl thel upper mrlt -by diitet visio llmo 1t1tlhr ma}y
be scn: tbrlttu'rou" te- low I.ter pr''t by refi:t-ion fhot'l- t5\ he tw:ro
m}IIiIrrolr. I Thle inlsrtitu nlt is'o eont trI, ivNcl.d, thla.t hwen; t Il ildex Is
m:oved u tl), where the 7elro-poit its placed, or the gradntation beati s the ttwo ireflectors i at d Itw a:e xaetly })arallel to
each other. If bwe no look through the telescope, T, so poctnt
ed nas to suee tghe star S tlrowu (lat t trtan parenltct ipart, of the:orizonl'glass te shart:ll seo te csarme star, w in,. t same place,'
reflected from itle silveret part; for the star (or anty sitilwr
O1ject) -is at suct: a distahn.e lthat the ray.s of liglht \helich strike
upon t.he index glass I, are parallel *to those which entel tlhe