Part 8

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.

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,

REPORT:—

That the following description of these telescopes is given by Mr. Holcomb:

The two reflecting telescopes now submitted by the subscriber, are constructed on the plan of Sir William Herschel, having the front view. The largest has a focal length of 9½ feet; the diameter of the speculum is 8½ inches, and has five astronomical eye-pieces, and one terrestrial eye-piece, for showing objects erect; the lowest power is 57, the highest 900. The smallest has a focal length of 7 feet 9 inches; the diameter of the speculum is 6½ inches, and has one terrestrial, and four astronomical, eye-pieces; the lowest power is 60, the highest is 600. They are of the same kind as those that were submitted a year ago, except the manner of mounting, which is very different.

AMASA HOLCOMB.

On the evening of the 4th of May, the committee met, by appointment, in an open lot south of the Pennsylvania Hospital, for the purpose of testing the performance of the telescopes, which had previously been tried by some of the members of the committee, and by other gentlemen, on the evening of the 2d. The result of the examination was highly creditable to Mr. Holcomb, and cannot fail to gratify all who have at heart the advancement of astronomical science in this country.

The instruments, with powers varying from 50 to 600 in the smaller, and to 900 in the larger, gave satisfactory views of the moon with a sufficiency of light.

Mr. Holcomb’s ability to manufacture telescopes which should bear a comparison, on favourable terms, with the best four and five feet achromatics now in the country, having been established by the report of the committee in May, 1834, their attention was chiefly directed to ascertaining the degree of perfection to which he has attained in his art, by his persevering efforts during the past year. Accordingly, the remarks which follow are made with reference to the larger telescope, of about ten feet focal length, eight inches aperture, and with a positive eye-piece, giving a power of about 900, and the surface of the field of view nearly twice as great as that of a Gregorian, and one-third greater than that of an achromatic telescope, under similar circumstances.

The view of the moon with its rugged surface, its ridges of mountains, and the endless variety of indentations on its surface, was interesting beyond description, and exceeded any thing of the kind the committee have ever witnessed.

Saturn’s ring, though not in a favourable position, was seen manifestly double, for the first time in this country, as far as the information of the committee extends.

The companion of Polaris appeared as a star of the fourth or fifth magnitude, to the unassisted eye.

The double stars, Castor, μ Draconis, 4 and 5 ε Lyræ, and 44 Bootes, were distinctly separated, and the dark space between them made evident. The last mentioned, consisting of two stars of the fifth magnitude, distant 3″ made a fine appearance; they were soft, and well defined, and there were no scattering rays of light, as was the case with Castor, in both instruments.

A class of closer doubles stars, of which 6 Coronæ, distant 1″.2., and ζ Bootes, distant 1″.4., may serve as examples, was acknowledged by the artist, last year, to be too difficult for his telescope. This has furnished a stimulus for his exertions, and the complete division of the latter, as witnessed by the committee on the present occasion, has been the reward of his disinterested labours. The discs of the two stars in ζ Bootes appeared to be tangent to each other. The committee have no evidence that the same has been effected by any other telescope in the country.

For the purpose of finding the limit to the power of Mr. Holcomb’s telescope, the committee called his attention to a class of still closer stars; among them were mentioned, ζ Cancri, μ2 Bootes, ῎ Coronæ, 36 Andromedæ, and ε Arietis, the last of which is only divisible by two telescopes now in use, viz.: the Dorpat telescope, and the twenty foot reflector of Sir John Herschel. These stars, distant from 0″.6. to 1″.0., are made to appear with their discs tangent to each other in those celebrated instruments, as appears by their notes appended to the observations contained in their printed catalogues. It is almost needless to add, that Mr. Holcomb acknowledged these stars to be too difficult for any telescopes he has yet made.

It may seem presumptuous to compare the small instrument of Holcomb with the chefs d’oeuvre of British and German genius; but, thanks to the admirable labours of the Herschels, of Struve, and of South, observers are enabled, through their printed catalogues, to compare together the optical capacities of their telescopes in distant regions. Accordingly it appears from an examination of these catalogues, and of Holcomb’s instruments, that what the best telescopes in Europe can do upon stars distant 0″.6., can be done upon stars distant 1″.4., by instruments which are the work of an unassisted, and almost neglected, American optician.

Judging from the progress made in his art, by Mr. Holcomb, during the past year, the committee look forward, with confident expectation, to the not far distant period, when, should his health be spared, the country will be in possession of a twenty feet reflector, of native workmanship, rivalling the best European instruments, and that, too, without the patronage of any corporate institution, should all of them be willing to waive the opportunity of sharing with him the merit of such an enterprise.

The committee have been led to enlarge upon this subject, from a knowledge that one of our national institutions has, within a few years, imported into the country, at an expense of $2,500, a telescope which, though excellent in its kind, is inferior to that exhibited by Mr. Holcomb, which was made and mounted to order for an individual in Georgia, at less than the eighth part of the above mentioned sum. It is not probable that a twenty foot instrument from Mr. Holcomb, would cost eight times as much as one of the length of ten feet.

The mode of mounting the instrument appears to be original, and nothing can exceed it in simplicity, or steadiness. Indeed, with a power of 900, no inconvenience was perceived from resting with one hand on the frame, and another on the tube, although the same could not be done with the mounting used by Mr. Holcomb last year, or with that of common achromatics with a power of 200, without serious inconvenience.

In conclusion, the committee beg leave to recommend Mr. Holcomb to the Board of Managers of the Franklin Institute, as a candidate for a premium and medal from the Scott’s legacy fund, for his new mode of mounting reflecting telescopes.

By order of the committee.

WILLIAM HAMILTON, Actuary.