The times. [volume] (Washington [D.C.]) 1897-1901, 1901-01-13

and safely was wOrk OF no hSHt NATURE
but he successfully accomplished it and
! eventually both mirror and the telescope.
| which weighed now altogether fourteen
| tons, were so well counterpoised that they
| could be easily moved in a limited direc-
ton by means of windlass worked by
| two men. The perfection of the seeing
| qualities of this instrument and its enor-
nous light-grasping powers were partic-
! ularly striking, and observational astron-
orny was considerably enriched by the dis
| coveries made with it.
Speculum metal was not destined TO
| stay; ten years later GS5O the genius of
Leon FoucauIt introduced glass mirrors
with a thin coating of silver deposited
chemically. and these have now universal
ly superseded the metallic ones.
The long supremacy of Germany in the
matter Of refractors was broken down ul-
tlmately by the famous English opdcian
and engineer, Thomas Cooke of York. His
| first considerable instrument, one of seven
inches aperture, was finished in ISL; and
| in 1865. a year before his lamented death,
he completed the first of our present giant
refractors one of twenty-five inches aper-
| ture for Mr.. Newall of Gateshead. In
consequence of the SUCCESS of Cookies
| achievement other large. refractors were
| soon undertaken.
Avian Clarke. the famous oDtician of
| Cambridgeport, Mass, at once commenced
a twenty-six inch for the Washington OBJ
servatory. The next was one of twenty
seven inches. made by Grubb for the
Vienna Observatory. Object glasses now
grew inch by inch in size, depending on
the increased dimensions of disks that
could be satisfactorily cast Gautier of
Paris completed a wenty-nine-and-a-haif
| inch for the Nice Observatory, while AL
van Clarke made an objective OF thirty
| inches for PulkoH a. In 1887 the latter sue
cessfutly completed the mounting Of an
objective of thirty-six inches for the Lick
Observatory. but this immense tense was
only achieved after a great number of
failures. Even this large object glass was
| surpassed in size by the completion in 1892
| of the forty-ineh which he made for the
Yerkes Observatory. and by that made by
| Gautier for the Paris Exhibition of the
present year.
So much, then, for the largest refrac-
| tors. In recent years, since the introduce
| ton of the silver on glass mirrors. with
| their stability of figure and brilliant sur
face, which can be easily renewed, re-
lectors of large apertures are again being
produced. The first of these was one of
| thirty-six inches aperture made by Calver
| for DR.. Common, who demonstrated its
| fine qualities and his own skill by the
beautiful photographs of the nebula of
| Orion he was enabled to secure with 1t.
Dr. Common himself has since turned his
| attention to the making and silvering of
large mirrors of this kind, and the largest
| he has actually completed and mounted
| equatorial!y Is one with diameter of five
feet Another of thirty-six inches aper-
| ture is in Use at the Solar Physics OBJ
| servatory.
| The progress of depositing silver on
| glass has led of late years to important
| developments in which plane mirrors are
used Foucault was the first to utilize
! such mirrors in his ''siderostat,' in which
| such mirror is made to move in front of
a horizontal fixed telescope which may be
| of any focal length, and no expensive
dome or rising fioog is required. The plane
mirror of the siderostat in the Paris Ex-
| hibition telescope is six feet in diameter.
! A variation of this instrument is the
coelostat more recently advocated by
! Lippnann. The Coutie equatorial mount
! Ing also depends upon the use of pane
mirrors; with such telescope the OBJ
| server is at rest at a fixed eyepiece or
! camera in a room which may be kept at
| any temperature.
Now that in astronomical work eye OBJ
servations are indlspensably supplemented
by the employment of photography, an
| important modification of the refracting
| telescope has become necessary; this was
| first suggested by Rutherford.
The ordinary achromatic object glass
consists, as a rule, of two lenses, one made
| of flint and the other of crown glass: but
| in this form the Photographic rays are
not brought to the same focus as the Wis
| uai rays This however can be achieved
| by employing three lenses instead of two,
each of different kinds of glass The most
modern improvement in the telescope is
| due to Mr.. Dennis Taylor of Cooke &
Sons, and to DR.. Schott and Prof. Able,
| whose researches in the manufacture of
| old and new varieties OF optical glass have
| rendered Mr.. Taylor's results feasible. By
| the Taylor lens outstanding color is abol-
| fshed. all the rays being brought abso-
| luteIy to the same focus. Such lenses
| can, therefore. be used either for visual
observations or for photography for spec
troscopy-
SPECTRoSCoPIC ASTRONOMY.
The branch of physics which at the pres
| ent day has assumed such mighty and
| far-reaching proportions in astronomical
| work is that dealing with spectrum anaIy-
sis, which although suggested as early
as the time of Kepler. did not receive any
impetus as regards its application to ce-
| lestial bodies until the beginning of the
present century at the hands of Wollaston
| and F'raunhofer. Then, however. it still
| lacked the chemical touch supplied after
| ward by Kirchoff and Bunsen. They
| when the light from any heated body is
passed through a prism is an index to the
| chemical composition of the light source:
the constitution Of a yapor, when in
condition to absorb light, can be deter
mined by an extension Of the same prinef-
pie, first demonstrated by Stokes, Ang-
strom and Balfour Stewart when the CEN
fury was about half completed.
The first celestial body toward which the
| spectroscope was turned was our central
luminary, the sun.
Wollaston first discovered that its spec
trum was crossed by a few dark lines; we
learned next from Fraunhofer, who in 1914
worked with instruments of greater pow
er, that the solar spectrum was crossed