St. Tammany farmer. [volume] (Covington, La.) 1874-current, 1890-02-15
CELESTIAL PHOTOGRAPHY.


iaterestlng FaaGs Explained 6y tie Astsoa.
Omer Royal for ireland.
dare say that . number Of my read
ers will be surprised TO learn that the
heavens contain multitudes of invisible
stars. There are, Of course, . few
thousand stars that can be seen by the
unaided eye, there are many millions Of
stars that can be seen with the aid Of .
great telescope, but over and above all
these arc . myriad Of stars that not
even the keenest Of eyes applied to the
biggest and most powerful Of telescopes
will show . trace of. Does not this
seem rather . puxsle. II wo can, not
see the stars under any circumstances.
how do we know that they are there?
Most assuredly we can not feel their
warmth and - the stars. are always ST.
lent they can make no impression on
our sense Of hearing. while our two Nc
manning senses Of taste and Of smell
are obviously not Of the slightest as-
sistance in the study Of the heavens.
Yet, Notwithstanding that We fail to
perceive these stars directly, We have
the most undoubted evidence Of their
existence.
A star may be 20 faint. that the beam
Of light it emits Is too feeble to render
it visible even with all the aid that
great telescopes can give. The instru-
ment collects together tle rays and con-
centrates them 50 th,t they are all
brought to q point On the sensitive
retina at the back Of Che eye. II the
rays happen to he suhlsiently powerful
to act on the very delicate nerves of the
eye message is sent from the eye to
the brain and the star ID seen. II, how
ever, the rays are too weak to excite tho
retina, then the star is Of course not per-
ceived. Under these circumstances, the
only hops OF discovering the star Is TO
substitute for the retina something
which shall be more sensitive to light.
1t has been found that the rays from
one Of these very faint stars when con-
centrated ON point Of . good photo
graphic plate will succeed in making
an impression, provided that suf-
acient exposure has been given.
The image on this plate can be
developed and then the star is revealed.
1t can therefore bc readily imagined
that the use Of photography IL now es-
sential to astronomical progress. It AZ
fords us the means Of depicting the
magnitudes and the places Of stars
which must otherwise have been quite
unknown to US.
Some one may feel desirous to ask tho
same question IS was put to me by IN
intelligent lad when was describing
10 him the way we proceeded: ''How
can you take photographs Of the stars"
he said, '.for you can only sec them at
night when It is quite dark, and yet
every body knows that photographs can
only be obtained when there is plenty
Of light" To explain this diflculty
you must understand what goes on in
side the camera when the operation is
in progress. The glass lenses produce
on the plate IN image OF picture OF the.
object to be represented. In bright day
light it is the reflected sunlight which
the object and enables it
to form the picture. II the object
has light Of its own, rs star has, then
this light will form the necessary i'm
age on the plate and . photograph may
he obtained. 1t is. however, essential
that all light from every source except
that coming directly from the object IT.
self shall be carefully excluded, for this
would act ON the plate and obliterste
the image. Indeed, when taking pho
tographs Of the stars we have 10 take
special precautions to exclude stray
light have heard of plate having
been spoiled by the fact that while It
was being removed from the telescope
after the stars had recorded their inn
preston = carriage drove up the avenue
and fash from the carriage lamp
CHANGED TO fall on the plate and com
pletely destroyed the faint images Of
the stars. Of course when once the
plate has been developed it is then as
as photograph taken in tho
way.-Sir Robert s. Bell, in