The Portland daily press., 1896-05-08
THE TELESCOPE.
A Discovery that Will Add Ten Per Cent to
Its Power.

ANS" AAN VIZ ~aA0J = - AOAs ~ Ne 4Aouu-
ings of the Yale Scientific School has
made discovery, the result of which is
new type of telescope. in which the
defect known as the secondary color
alteration is removed without the use of
other than the ordinary silicate glasses.
The telescopes of the 18th century con-
sisted of a single lens as the object glass,
and in the hands of Huygens, Campani
and Davani reached the highest excellence
attained for the simple telescope. They
found that with increasing power it.was
necessary to increase the length of the
telescope in a very rapid ratio. This
fixed q limit to the practicable dimen-
sion. One telescope WAS actually made
by Azout, French optician, which was
800 feet long, but could not even be set
up.
Not until Sir Isaac Newton's discovery
of the law of dispersion of light was the
explanation of this inconvenient fact
found in the variation of focal length or
the varying wave length OF light. This
defect has since been called chromatic
oDOIIdUltu, bAC chCGu VI WalCu W OuG
looking through the ordinary telescope,
to show objects in prismatic colors, and
hence indistinctly.
Newton's Investigations led him to the
conclusion that this defect could not be
remedied, and therefore he turned his
attention to the rejecting telescope, the
first one of which he constructed withlhis
own hands. This kind of telescope be-
| came the type or instrument of tie eigh-
tenth century. The conclusion of New
ton was unquestionable until after the
middle of the last century when Dolland
was led to dissent from it. His researches
led to his invention of the object glass
made of two kinds of glass by which
most of the ohromatic alteration was
eliminated. This constitutes the modern
| so called ''achromatlc' telescope, In
which, however, the correction Of color
alteration is not perfect, as no two kinds
of glass have been found which will ex-
actly correct each other. The color left
over in this telescope construction is
called secondary chromatio alteration.
Up to the middle of the present century
this did not prove a very serious defect.
But with the increasing size of tele-
scopes and the applications of astronomi-
| cal photography, and particularly in spec
trosoopic work, sthe defect has proved very
yexatious-in photography because tele-
sope thus corrected for visual purposes
is worthless, and, conversely, one which
is adapted to photography is useless for
observations by the eye. Then, too,
just as in the old simple telescope, the
secondary color defect demands an in
| creaEing ratio Of length with increasing
power, and thus greatly adds to the en-
gineering obstacles to mounting large in
struments. Again, in spectrosoopy the
nf totl, Pow inn
wave lengths is such an Inconvenience
that many spectroscopes are provided
| with three pairs of object glasses correct
| ed for different regions Of the spectrum,
in order partly to eliminate the trouble.
In developing last summer the optical
equations involving the thickness and
! separation of lenses to the second order
| term which might be Of the opposite sign
| to that involved in the equations for color
correction. Although it seemed improba.
| BILE that this would afford means OF
correcting the old error, it demanded, In
the professors opinion. further investiga.
ton. After much labor he demonstrate d
theoretically a new method by which the
secondary ohromatic alteration, which
had resisted solution for almost century
and q half, might be remedied. He next
constructed a telescope with ratio Of
focal length to diameter of only 1-2.
for use with the spectroscope This has
fulfilled IN every way the hopes founded
upon the theoretical investigation. II
shows the solar spectrum with absolutely
unvarying focus from extreme red to OK.
treme violet, eliminating all secondary
color alteration. While the experiment
has not gone beyond this, there is nc
reason to doubt that the method is appli.
cal bic to telescopes of all sizes.
Several years ago Prot. Hastings pub.