Evening star. [volume] (Washington, D.C.) 1854-1972, 1896-05-08
NEW TELESCOPE DISCOVERED.
Defect of Secondary Color Alteration
Removed Without Special Lenses

From the New York Evening Post.
A very important discovery has been
made by Prof. c. S. Hastings of the Yale
Scientific School, the result of which is a
new type Of telescope. in which the defect
known as the secondary color aberra ton
is removed without the use of other than
the ordinary silicate glasses.
The telescopes of the seventeenth century
consisted of single lens as the object glass
and in the hands Of Huygens Campani
and Darini reached the highest excellence
attained COR the simple telescope They
found that with increasing power it was
necessary to increase the length of the tele-
scope in z very rapid ratio. This fixed a
limit to the practicable dimension One tele-
scope was actually made by About a
French optician, which was GO feet long
put 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 vari-
alton of focal length for the varying wave
length OF Light. This defect has since been
called chromatic alteration, the effect of
which is, TO one looking through the ordi-
nary telescope, to show objects in prisma tic
colors, and hence indistinctly.
But with the increasing size of telescopes
and til applications of astronomical pho
Biography, and particularly in spectroscopic
work, the defect has proved very veXatious
in photography because a telescope 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 in the old sim
pie telescope. the secondary color defect
demands an increasing ratio of length with
increasing power, and thus greatly adds TO
the engineering obstacles to mounting large
instruments. Again, in Spectroscopy the
variation of focal length for varying wave
lengths is such an inconvenience that many
spectroscopes are provided with three pairs
of object glasses corrected for different
regions of the spectrum, in order partly to
eliminate the trouble.
In developing last summer the optical
equations involving the thickness and sep
aration of lenses to the second order of
magnitudes, Prof Hastings found q term
which might be of the opposite sign to that
involved in the equations for color correc-
ton. Although 1t seemed improbable that
this would afford means of correcting the
old error, it demanded, in the professor S
opinion, further investigation. After much
labor he demonstrated theoretically a new
r:ethod by which the secondary chromatic
alteration, which had resisted solution for
almost century and half. might be rem
edied. He next constructed a telescope
with a ratio of focal length to diameter of
only 81, for use with the spectroscope.
This has fulfilled in every way the hopes
founded upon the theoretical investigation
1t shows the solar spectrum with absolute
ly unvarying focus from extreme red to ex-
treme violet, eliminating all secondary color
alteration. While the experiment has not
gone beyond this, there is no reason to
doubt that the method is applicable to tele-
scopes of all sizes.
at least 10 per cent to the power of the tel
escope, so that an instrument with 10
inch object glass will be about equal to an
11-inch telescope of the existing type