Mr. Herschel on Reflecting Telescopes.

289

of the superb equatorial, constructed by him for the Observatory at Dorpat. The existence of such an instrument in such hands ought to be a matter of congratulation to every lover of science, and I am rejoiced to hear from Mr. Fraunhofer, that, so far from resting satisfied with what he has already achieved, he has extended his views to the construction of still more powerful instruments, and is actually employed on an equatorial of 12 inches aperture and 18 feet focal length, which will indeed deserve to be regarded as a chef d'œuvre of art.

I have not the least disposition to doubt the merits of the Dorpat telescope, which I have not seen, but which I am disposed to believe one of the best, probably the very best refracting telescope, ever made. The accounts which Mr. Struve has been so

kind as to transmit me of its extraordinary power are quite satisfactory. A telescope capable of easily separating the stars of w Leonis, and giving exact and coincident measures of their rela tive position on several nights, can admit of no question as to its excellence. I have hitherto heard but of one other refracting telescope in which this star has been seen double. It is that by Lerebours, now at the Royal Observatory at Paris, the object glass of which (like the Dorpat telescope) is 9 inches in aperture, though only 8 are effectively employed. Mr. Struve, however, has dis covered double stars still closer than w Leonis with his magnificent instrument.

My immediate object in writing this, however, is to obviate an erroneous impression which may arise in the minds of those who read Mr. Fraunhofer's memoir, as to the great inferiority of reflect> ing telescopes in point of optical power, to achromatics in general, and more especially to those constructed with such delicacy as his own doubtless are. Those who have witnessed the performance of Mr. Amici's beautiful Newtonian reflectors, will not readily admit this inferiority, but will rather feel disposed to wish that some attempt might be made to accommodate such admirable instruments to the more exact purposes of astronomy, an object which appears to have been too easily lost sight of.

Mr. Fraunhofer's expressions, when speaking of the loss of light

288

On the Large Achromatic at Paris.

object glass actually in use is in proportion to Mr. Struve's (if all of it be effective) as 70 to 92 nearly; a difference not very hard to be allowed for. I have seen the nebula in Orion; the planets Jupiter and Saturn with the Paris telescope; and with their appearances in Mr. Herschel's 20 feet reflector I am perfectly familiar, and the comparison is many times in favour of the latter.

The power of the 20 feet reflector at Slough is well authenticated; and if the indefatigable astronomer of Dorpat will turn his probably matchless achromatic upon some of the faint nebulæ in the constellation Virgo, or upon some others not easily resolvable into stars, he will soon satisfy himself, that his ideas of its spacepenetrating power are much overrated.

JAMES SOUTH.

P. S.-The star & Bootis was seen" close double" by Mr. Pond at Lisbon, perhaps 20 years ago, and, as I believe, with a Newtonian reflector of 6 inches aperture; and the circumstances mentioned in a letter written by him to Dr. Wollaston. The instrument with which I first observed it, in 1810, " close double," was a reflector of the worst possible construction, viz., a Gregorian reflector of 6 inches aperture and 30 inches focal length, but a very perfect instrument made for me by Mr. Watson in the year 1809, and which is now in the possession of my friend, Mr. Frederick Perkins.

I am happy to say that I have observed Encke's comet, and also the comet of the constellation Taurus four successive nights, viz., August 21, 22, 23 and 24.

ART. VIII. Observations on Mr. Fraunhofer's Memoir on the inferiority of Reflecting Telescopes when compared with Refractors.

Extract of a Letter from J. F. W. Herschel, Esq., F.R.S., to M. Schumacher.

Dear Sir,

Slough, 1825, Aug. 15. In the 74th Number of your Astronomische Nachrichten, you have obliged the astronomical world with Mr. Fraunhofer's account

On the Large Achromatic at Paris.

287

420, which could not be overlooked by the most careless observer, and with 560 both stars were admirably defined. Measures of position and of distance might have been gotten with the greatest facility, but for want of a micrometrical apparatus. It was my in tention to have submitted the instrument, which was placed at my disposal by our amiable and ever-toiling friend, Monsr. Bouvard, in the most unreserved manner possible, to other and more severe tests; but in elevating the telescope to Bootis, the stand becamė deranged, and the instrument was rendered unmanageable. On a subsequent night, a similar accident also foiled us in our attempts to investigate its power.

I need not inform you, that a telescope having an object glass of the diameter above mentioned, which with these powers will neatly define the limb of the planet Venus, and will give to the discs of the double stars here named, images absolutely round, deserves to be well spoken of. Indeed I have no hesitation in saying, that this telescope is the best achromatic I ever pointed to the heavens; nor will I withhold my regret, or even the mortification I feel in asserting, that England, when I visited it in May last, could not produce an achromatic any thing like it. The stand upon which it is mounted is not provided with any means of giving to the telescope equatorial motion. I hope, however, that the Board of Longitude of Paris, in their accustomed zeal for the promotion of astronomical science, will, ere long, render this noble instrument more available to the purposes of scientific research, by voting to it the indispensable attributes of an equa.I torial. Gambeys' mechanical head would soon convert it into an instrument, which would be worthy of the French nation.

Whilst, however, I say thus much, I am far from entertaining the sentiments of Mr. Fraunhofer as to the decided superiority of refractors over reflectors; nor can I accompany Mr. Struve in his idea that the Dorpat telescope "may perhaps rank with the most celebrated of all reflecting telescopes, namely, Herschel's;" it is true, I have not had the enviable gratification of having seen the former; still I think the Paris telescope furnishes me with data, upon which to form something like a rational conjecture. Its

X 2

ART. VII. Examination of the Large Achromatic of the Royal Observatory at Paris.

Extract of a Letter from James South, Esq., F. R. S., to

Dear Sir,

M. Schumacher.

Passy, près Paris, 1825, Sept. 6. I have the honour to transmit you the enclosed which I have received from Mr. Herschel. I have hitherto been prevented from doing much in the way of ordinary astrónomy, by a resolution to complete my observations of the double stars; and I am glad to say, that labours in this department of astronomical science are (at least for the present) fast drawing to a close. By the latter part of October next, I have little doubt but that my observatory at this place will be dismantled; and it is my intention to present the observations of 400 additional double stars to the Royal Society, at its first meeting in November.

my

From your Astronomische Nachrichten, and subsequently from

the Memoirs of the Astronomical Society of London, I am glad to find that the Dorpat Achromatic realizes the expectations of its possessor; and I rejoice very heartily that it has fallen into such hands.

Mr. Struve will not allow it to remain idle.

In the accompanying communication from Mr. Herschel, allusion is made to the achromatic constructed by Lerebours, now placed in, and belonging to, the Royal Observatory of Paris; perhaps a few lines devoted to it, will not be altogether unin

teresting.

The diameter of its object glass uncovered by the cell (in English measure), is rather better than 9.2 inches, of which 8.4 inches only are in actual use; its focal length is 11 feet. The magnifying powers with which I used it, on the night of the 15th of March last, are 136. 153. 224. 240. 420. and 560. With all except 560 (which by some forgetfulness was not applied,) Venus was extremely well defined during dark night; of course, Jupiter and Saturn were well shewn. The two stars of Castor, of y Leonis, of Orionis, were exhibited with 240, 420, and 560 as round as possible: Leonis presented by its side a light blue star with

Tyrant Shrikes of America.

Griseo-fuscus, infrà flavus; remige primo sub-acuminato
Olivaceo-griseus, infrà flavus; mento albo; caudâ nigrâ

Nigro-cinereus, infrà albus; rectricium apicibus albis

leucotis

Parvus ; fuscus, albens infrà; temporibus albis, strigâ oculari nigrâ
Griseo-fuscus, flavus infrà; jugulo cinereo; remige primo obsolete accuminato ferox