The moon, nearly full, was at a height to be conveniently viewed with
the lower powers of the instruments: with a power of 350 in the five
feet achromatic, the moon appeared bright and well defined,—with the
same eye-piece, giving a power of 400, in the reflector by Mr. Holcomb,
the moon was sufficiently bright, and equally well defined. The same,
with the exception that the moon was more brilliant, and the field of
view much greater, was remarked with the use of Mr. Holcomb’s highest
magnifier, giving a power of two hundred and fifty.
As an illustration of their comparative performances, it was remarked
that the waved appearance of the outer declivities of the craters of some
of the apparently extinct lunar volcanoes, indicating the successive
depositions of the lava, was more manifest with a power of four hundred
in the reflector.
The immersions of 3 and 4 Geminorum of the sixth and seventh magnitude,
were observed at the same instant of time in each.
The same occurred the evening before with a star of the eighth or ninth
magnitude.
The immersions, however, of two very small stars, apparently of the tenth
or eleventh magnitude, were observed with difficulty in the refractor,
but could not be observed at all in the reflector.
The comparison of Polaris was best seen when the moon was up in the
refractor, but in the absence of the moon it was readily seen in both.
Castor was easily divided with the lower powers of either, but in the
case of this, as well as of other binary and double stars, the dark space
between the stars was less disturbed by scattering rays in the reflector
than in the refractor.
ε Bootes was seen double in each, but more distinctly in the reflector, μ
Draconis, γ Leonis, and 4th and 5th ε Lyra, were seen distinctly double
in both instruments; μ Draconis, from the equality of the disks and
softness of light, presented the finest appearance.
γ Virginis, with a power of three hundred and fifty in either telescope,
gave no certain indications of being double. Some of the members of the
committee were of opinion that it was slightly elongated.
It was stated by the artist that his reflector would divide stars distant
3″ from each other.
Estimating the distance of the stars observed by the late observations of
South, Struve, and Herschel, jr., the committee were of opinion that his
instrument is adequate to the distinct division of double stars distant
from each other 2″.5.
The motion of this instrument, plainly mounted, was steady, and with the
finder, even without rack work, objects were easily made to range with
the centre, or line of collimation of the instrument.
The position of the observers with the Herschelian telescope, was natural
and easy in contemplating objects having seventy or eighty degrees
of altitude, though quite constrained and inconvenient in using the
achromatic.
The reflector gave a distinct view of land objects, even when within
one-fourth of a mile.
Some light was lost by the position of the head, an inconvenience
partially obviated by making the end nearest the object three inches
greater in aperture.
The Gregorian, which probably was not a very fine instrument of its kind,
bore no comparison in distinctness, or in quantity of light, with the
Herschelian telescope.
From these trials, the committee are of opinion that Mr. Holcomb has been
entirely successful in the difficult art of polishing specula with the
true curve, which gives to the objects viewed all the distinctness of
figure that is given them by the best refractors by Dolland.
In one respect, the largeness of the field of view, the reflectors by
Mr. Holcomb have a decided advantage over achromatics and reflectors of
different construction. The apparent diameter of the field of view in
the Herschelian being nearly double that of either, with equal freedom
from aberration. The quantity of light furnished by the refractor was
greater with the same aperture, an important advantage in searching
for, and observing very minute objects. This deficiency of light in the
Herschelian for viewing faint objects near the moon, or satellites near
their primaries, the committee are of opinion may be removed by enlarging
the aperture of the Herschelian reflector to five or five and a half
inches.
The simplicity of the method of preparing and mounting Mr. Holcomb’s
telescopes is worthy of notice, since on this plan, the artist is enabled
to furnish for an expense of one hundred dollars, with plain mounting,
or of one hundred and fifty to two hundred dollars, with more expensive
mounting, telescopes whose performance equals that of Gregorians and
achromatics hitherto imported into the country at an expense of five
hundred dollars.
By order of the committee.
WILLIAM HAMILTON, _Actuary_.
_May 8th, 1834._
_Report on Holcomb’s Reflecting Telescopes._
[From _Journal of the Franklin Institute_, July 1835, new ser. vol. 16
(whole no. 20), pp. 11-13]
The Committee on Science and the Arts, constituted by the
Franklin Institute of the State of Pennsylvania for the
promotion of the Mechanic Arts, to whom was referred for
examination, two reflecting telescopes, made by Mr. Amasa
Holcomb, of Southwick, Hampden county, Massachusetts,