New-York tribune., 1880-01-11
SCIENCE FOR THE PEOPLE.
MAKING A SILVER REFLECTOR.


Mr.. Calyer, the maker Of tho 37in. silver
on-glass reflector for Mr.. Common Of Ealing,
England, an instrument which was the first TO
detect the satellites of Mars last year, anu
said to be one of the most perfect rehectors IN
existence, describes in the monthly notices OF
the ROYAL Astl onomical Society how the spec,
ulum was made. The disc was ground ana
polished with tool 36ln. In diameter, anu
weighing nearly 9cwt., and the f0cus-ln..
712in. was within an inch or two OF what was
intended. .,
The machine / employed, says Mr.; Calyer,
was on a principle which was 9 modlncatlon
of that used by the Earl OF Rosse. nave
made five different machines, one OF Whicn
was on the principle used by MIR. LasseilEan
excellent principle. But l have long an srnSo &c
the final work like the trained hand, ana .,s was
gratifed to learn, when in conversation WIN
Professor Draper. of New York, that MS eA-
perience agreed precisely with my own on
this point. The fact that, when polishing
specula, we use a tool that is somewhat elastic
(for there is nothing equal to pitch as q ma
terial for the pOlisher) shows that IL is the
form of the polisher we want TO ann at' anu
the curve of the glass will follow. ihereIoIee
the machine, its rates OF revolutions anu
strokes the size and weight OF the pollsners'
their consistency (Which depends on the teIn-
perature in which they are worked, anu tle
frictlOn) should all be arranged so as 10 give
to the polisher the figure or curve we desire
the 37in. disc, as an experimental one an
sWered all expectations. and lulhiled all hec-
essary conditions; and may also state, as the re-
suit of my experience. that see no obstacle 10 tne
construction of glass mirrors OF very large SIZES
Before polishing was commenced portion Oh
the workshop was covered with calico 10 ex-
clude dust room only being left for WORKING
wooden tunnel, commencing at the uo0r
and extending forty feet on the ground ievel,
was covered with sailclOth to Keep out ilght
and air-currents. The machine was maae 80
that its revolving table could be turned from
the horizontal to the vertical position the
speculum was worked In its cell the whoie
| weighing nearly 11cwt. When testing com
menced | sat in this dark tunnel to test at the
centre of curvature, and the spectrum was
| focussed by an assistant with screws, so that
| received in my eye the image of very
minnie pinhOle in q lamp screen close Oy my
head During the polishing and hgurng
| carefully studied the behavior OF the dlsc-iOi
| aexure-for distortion of figure by contraction
| and expansion during changes OF temperature
and | found the disc as perfect rs 6-lncn
one. and without single infirlnlty
The work of correcting was tedious ana Try
ing. especially Ill the latter stages, when fol
every few minutes polishing the whole prepa'
ratons for testing had to be repeated, ana tht
! settling of the mass into It's normal state Nat
| To be patiently waited for, and often days
passed before further advance could be made
! When the figure had advanced as FAR rs ncces
| saIy for testing at the centre OF curvature, tht
wooden structure was thrown open at tn'
outer end for the purpose OF testing on a dis
| tant terrestrial object. The test objects fol
| daylight purposes consisted OF hole punched
| through a sheet OF metal, with reflector 81
placed st the back as to reflect the ight OF th'
| clouds through, so that IT appeared as brgh
white spot a spot of whiting aud printed pa
per was used for definition. When the SUI
| shODe 1 obtained its image by means OF :
plism with one of its surfaces polished To
small spherical curve This was made use o
because could not use bulb, having 10 lOOl
| in the direction of the sun, and not with TN,
sun behind me. For testing at night lamI
covered with metal screen with a hOl,
| through IL was used. For every advance to
ward The correct figure il was first tested a
| the centre of curvature, and then on the dis
| tant object In the latter case ii was. o
course. used as a telescope proper, by placing
a plane at Its working focus for parallel rays
| The plan OF figuring was that of local figur
| ing and correcting. The polish and surface
were obtained with the large and heavy pol
| isher, and corrected with a number of polish.
| ers of various sizes and forms TO suit every
stage Of the progress and the temperature OF
the air, etc. II an error of iucgularity OF
| figure set in. it was polished out with th'
large polsher. The disc sIlvered 10y
! Martins process. 1t was we'll svered II
twenty minutes, then well washed and spongco
| over with a handful OF cotton wool, finally
rInsed, left to dry and polish. 1t was con
veniently silvered with the help of one assist