Evening star. [volume] (Washington, D.C.) 1854-1972, 1910-05-01

selves with by making 'rainbows." But
each surface OF this prism has been
fashioned, with the utmost skill and &C
curacy, into an exact plane This achieve
ment would appear. perhaps to many
persons as simple. The possessor of an
ordinary looking glass. If the latter is
free from that 'wavy' appearance which
is apt to produce such tragic uncertainty
in the operation of shaving is likely to
plume himself upon the fact that the
surface of his mirror is quite smooth or
plane But such. OFFICIALLY speaking, is
very far from the fact
The mirror and finest glass surface OF
commerce would be found. upon exami-
nation by means of optical tests, to be
extremely rough and uneven, and to pro-
duce reflections and images which, tor
scientific purposes, would be very mac
curate and totally useless. 1t is the pro-
duction of this perfectly plane surface,
which is unknown to the ordinary article
of commerce. that so wonderfully in
creases the intrinsic value of the lump of
salt.
Inc employment of this curious ma
terial, in the first place. for optical pur-
poses, Mr.. kahler explains, is due to the
TACT that 1t admits the passage of the
heat rays of the sun much more readily
than glass. AS is well known. the heat
rays of the luminary appear at the red
END or the latters spectrum. Indeed thew
extend for considerable distance beyond
the visible end of this portion of the
solar spectrum, as was demonstrates bv
Prof. Langley by means Of his boometer.
just as 90 the greater percentage of vio-
let rays elude human vision at the oppo-
site extremity of the spectrum
Now the present work of the Smith
sonlan astrophysical laboratory involves
an endeayor to obtain the most perfect
photographs possible of the solar spec
trum, that phenomenon that can be pro-
duced in degree by any one in a dark
prism But these rock-salt prisms.
which are made BY Mr.. Kahler for the
Smithsonian sun sharps, have such an
exceedingly sensitive surface that they
are kept IN Grass cases perfectly air
tight. as the slightest moisture or any
undue vibration would impair or de-
strOy their efficiency
Apparatus for sun photography ex-
traord1nary is, In short, what Mr. Kah-
Ter supplies for the scientists who work
mysteriously in the frame sheds back
of the Smithsonian building and at the
Mount Wilson observatory
This apparatus Involves with the rock
salt prislns described refiectors or mir
rors for the purpose of receiving the sun s
rays direct and throwing them I'm accurate
~ parallel lines upon the prisms, thus
forming the spectra from which the pho
tographs are made. The construction OF
delicate an undertaking as that of max
ing the prisms, for the surface is consid-
erably larger.
For making one q: these mirrors. in
VolVlng about month of continual labor
Mr.. kahler receives 81 400, in the case of
a glass of sixteen-inch diameter. The
last mirror of this sort supplied by Kah
Ter for the
mounted at the Mount Wilson ohserva-
tory. Is of eighteen-inch diameter and 10
made of special metallic composjtton
instead of glass. This new metal mirror
Is round 10 reflect certain rays of the sun
which have hitherto eluded observers anS
is an example of the marvellous nroerasa
ana possibilities in modern onticaI In
strunent construction IA
since the obtaining CT perfectly plane
or even surfaces in salt glass or metaI
is one OF these optical achievements
which have made Mr.. Kahler famous
among scientists the world over and
winch enable him to command compare
lively large sums OF money for his labor
1t may he asked how is he enabled to
know when he has attained this requisite
result TO perfection. The method of test
ing. which he kindly undertakes to il
lustrate to the visitor would annear to
be quite simple. though 1t is based upon
the most beautiful scientific principles.
The latter involve the fact that no mat
ter how perfectly the surfaces of two
planes be ground there intervenes he
tween them certain space, generally
ties ed at one two-hundred- and fifty GU
sandths Of an inch-of course, an incon-
ceivably minute interspace. This is call
ed the air-film, and is due to particles OF
dust which it is impossible to exclude.
A test plane, glass d sc, the surface of
which has been previously ascertained to
be perfect plane. and lamp producing
sodium rays. are all the paraphernalia
which Kahler requires to ascertain wheth-