"Now, I infer from the above, firstly, that the forms of silver mirrors can be produced with sufficient accuracy to afford the same precision of the image that is attained with refractors, and I thereby refute practically the opinion that reflectors never give as distinct images as refractors do, and, secondly, that a subsequent polishing of the mirror, which may become necessary, should it tarnish with time, has no appreciable influence on the form of the mirror. When one considers that these mirrors may be protected from tarnishing when they are not in use by a cover of india-rubber, so that a fresh polish will not have to be given to them above once a-year-object-glasses, too, must be cleaned at least once during that time-and that if after the lapse of many years the silver surface of the mirror should have suffered, it is a light matter to have it silvered again, the objection hitherto brought against catoptrical instruments on the score of their want of durability appears also to be overcome.

"Hence we now possess reflectors which are in every respect free from the faults of the former instruments. What advantages in the mechanical construction of astronomical instruments may not be attained if one may presuppose two reflexions! I will here only touch shortly upon two arrangements, treating this matter in detail in a monography, that the Bavarian Academy of Sciences will publish in the ensuing autumn on the occasion of its centenary anniversary.

66

Catoptrical meridian circle, or transit. The axis of the concave mirror of 6-inches aperture and 36-inches focal length, lies east and west, the mirror is fixed on a stone pillar. It receives light from a plane mirror, permanently fixed at an angle of 45°, and which rotates round the prolongation of the optical axis of the concave mirror. This axis of rotation, as also the plane mirror, is traversed by an aperture, so that the image of a point in the meridian falls in the axis of rotation. The cross wires are placed here, and the eye-piece sees them distinctly, as well also as the image of the point in the meridian. An altitude circle fixed firmly on the perforated axis, and lying in the plane of the meridian, measures the nadir distances from a quicksilver horizon. The spherical aberration of the marginal rays of this mirror amounts to o"374, and this is strictly corrected by means of a small object-glass, having a long negative focus, which is inserted in the tube 3 inches in front of the cross wires. If both the mirrors are attached to the same stand, pretty much in the manner shown in the figure, we have a reflecting telescope mounted horizontally, since a rotation about A alters the azimuth, while a rotation about B alters the altitude.

"If the axis A be made to coincide with the axis of rotation of the heavens, the telescope is equatoreally mounted, for rotation about A measures the hour-angle, about B the declination. Of course the eye-piece tube is provided with a finder,

60 Dr. Steinheil, on Silver Mirrors for Reflecting Telescopes.

for the sake of more readily meeting with the objects. This telescope possesses, moreover, another advantageous quality, the cost of an instrument of 6.4 inches aperture English measure

.

is only 300 Bavarian florins (about 257. 10s.), and in its action it must be superior to a refractor of equal aperture, because it is free from chromatic aberration. For this reason, especially, catoptrical heliometers will possess peculiar recommendations. I am convinced that by the employment of silver mirrors upon glass we shall arrive at obtaining reflecting telescopes of colossal dimensions. Only in that case it will be necessary to adopt a method of mounting them different to that now in use. It appears to be indispensable to give the telescope, or rather the two mirrors respectively, for the tube may be dispensed with, a constant position.

"If the axis of the concave mirror is brought to coincide with that of the rotation of the heavens, and if the plane mirror, the centre of which is perforated, is so disposed that it transmits to the concave mirror situated parallel to its axis, the parallel rays proceeding from a star, then can the image, formed by the concave mirror be observed through the aperture in the plane mirror, and the elimination of the diurnal rotation of the earth is reduced simply to a rotation of the plane mirror about the axis of the heavens. For other declinations one has only to give the plane mirror another inclination. When thus arranged no special observatory is needed for the erection of a telescope of 2-feet aperture. The plane mirror might be adapted in front of a window of a dwelling-house, the concave mirror being situated below on the ground in the direction of the axis of rotation of the heavens. A tube between the two mirrors would be quite superfluous.

"By means of this application instruments of great optical power, and simple and convenient to manage, are rendered accessible to the admirers of astronomy, and that, too, at a moderate outlay, which is a point that should not be by any means overlooked.

"Munich, April 5, 1858."

Catalogue of 317 Stars selected from the B.A. Catalogue (being such as were supposed to have large proper motions), deduced from Observations made at the Honourable E. I. Company's Observatory at Madras in the Years 1853-7. By Capt. W. S. Jacob, Director of the Madras Observatory.

"The instruments employed on the observations from which this Catalogue is derived are a 5-foot transit and 4-foot mural circle, both by Dollond, which have been frequently described in the Madras volumes. Nearly the whole work of the Catalogue, both as regards observation and reduction, was executed by the native assistants in the observatory, and will be found creditable to them. In case of any doubt or discrepancy, I have occasionally repeated an observation, and have, of course, maintained a strict watch over the errors and adjustments of the instruments; the whole of the reductions have also been carefully checked by myself.

"During a portion of the time embraced by these observations, the charge of the observatory devolved on Major W. K. Worster, of the Madras Artillery, in consequence of my absence in England for the recovery of my health.

"The large proper motions which had been assigned to many of the southern stars, are for the most part contradicted, and seem to have their origin from instrumental or other errors in the Paramatta observations, they having been dependent chiefly on the Brisbane Catalogue."

Proper Motions of the Stars of the Greenwich Catalogue of 1576 Stars for 1850 not included in the Greenwich TwelveYear Catalogue, deduced by Comparison with the Results of Bradley's Observations as given in the "Fundamenta Astronomia." By the Rev. R. Main, M.A.

This paper is a continuation of a former paper by the author, published in vol. xix. of the Memoirs of the Society, containing a determination of the proper motions of all the stars which are common to the Fundamenta and the Greenwich Twelve-Year Catalogue of 2156 Stars. The last-mentioned Catalogue, and the one referred to in the title of this paper, contain the results of all the star-observations made at Greenwich from the year 1836 to 1853 inclusive, comprising a period of eighteen years. The whole number of stars of which the proper motions are assigned in the two papers amounts to 1437. It was explained in the introduction to the former memoir that all the stars observed at Greenwich have been selected for special purposes, and that the accurate determination of their places forms one of the most important objects of modern sidereal astronomy. In this respect the stars contained in the

B

Catalogue of 1576 Stars are particularly valuable, several new classes having been introduced. Amongst these may be mentioned the results of the observations necessary to complete the list of all stars visible at Greenwich between the first and fourth magnitudes (with very rare and accidental exceptions); of a list of about 140 stars, to be used in addition to those given in the Nautical Almanac, for the determination of clock-error; of stars in the constellation Canis Major compared with Sirius for the purpose of more accurately determining the variability of the proper motion of the latter star; of about thirty stars near the north pole arranged in pairs differing by nearly 12 hours in right ascension, useful for determining azimuthal errors; and, finally, of such stars as have conspicuous proper motions.

Occultations of Stars by the Moon. Observed by Capt. Noble.

Friday, November 19th.

h m 8

Arietis. The star disappeared instantaneously at 4 22 18.8 L.S.T.
The reappearance was not seen.

Monday, November 22d.

h m

136 Tauri. The star disappeared at 2 8 37'5 L.S.T.

And reappeared at

3 13 38.8 L.S.T.

The star entered into contact somewhat sluggishly, and the brightness of the moon was almost overpowering; notwithstanding which the disappearance was well seen. The emersion was perfectly instantaneous.

The telescope employed in each case was my Ross equatoreal of 42 inches' aperture, with a power of 115, adjusted on the respective stars.

Observatory, Forest Lodge, Maresfield, Sussex,

9th December, 1858.

Physical Observations of Comet V., 1858. By
E. B. Powell, Esq.

Donati's Comet was first noticed at Madras on September 30th, when it was seen through breaks in the clouds shortly after sunset. On succeeding evenings it gained rapidly in brilliancy, becoming a truly magnificent object. My equatoreal not having arrived from England, I resolved to try how closely I could determine the comet's path by means of sextant observations, and accordingly took the following measures with

a good 8-inch instrument by Troughton and Simms, graduated on platinum to 10"; and I am in hopes that the care I expended upon the work will have prevented the error on any distance from rising above 1'.

The difference of longitude between Madras and Greenwich was taken at 5h 21.

On all the dates except November 4th, the observations for the star A were taken in two sets; one before the measures for the star B, the other after, in order that the mean distances might be reduced to the same instant. To clear the resulting right ascension and north polar distance of the comet from refraction, I calculated* the errors in R.A. and N.P.D. on the catalogue places of the stars for the moment of observation, added them to the true R.A. and N.P.D., then worked out the apparent R.A. and N.P.D. of the comet, and finally applied to these last the appropriate corrections. Pontécoulant's modification of Laplace's method of approximating to the perihelion distance, and the epoch of perihelion passage, were next applied to the observations of the 4th, 8th, and 12th October, and gave approximate values of the fundamental elements. Owing, however, to an error in the reduction of the distances of the comet from the stars of reference on the 12th Oct., which was not detected for three or four days, the values were much

* In this a table of refractions in hour-angle and north polar distance for Madras, drawn up by Capt. Jacob, proved of service.