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qualities of cheapness, portability, great stability, and most accurate and delicate adjustment were combined.

Four hundred and thirty-ninth meeting.

May 12th, 1857. — MONTHLY MEETING. The Academy met at their rooms. Professor Treadwell, Vice-President, in the chair.

The Corresponding Secretary read letters from the Ethnological Society, London; the Royal Saxon Society of Sciences, Leipsic; the Royal Bavarian Academy of Sciences, Munich; and the Boston Society of Natural History, acknowledging the receipt of the Academy's publications; and from the Royal Bohemian Society of Science, Prague, presenting its Transactions.

Mr. G. P. Bond communicated the results of an examination of the photographs of the star Mizar (§ Ursa Majoris), with its companion, and the neighboring star Alcor; specimens of which were exhibited.

“ Daguerreotype images of the star Vega (a Lyræ) were obtained at the Observatory of Harvard College by the well-known artist, Mr. J. A. Whipple of Boston, on the 17th of July, 1850, and subse. quently impressions were taken from the double star, Castor, exhibiting an elongated disc, but no distinct separation of its two components. These were the first, and, till very recently, the only known instances, of the application of photography to the delineation of the fixed stars.

“A serious difficulty was interposed to further progress by the want of suitable apparatus for communicating uniform sidereal motion to the telescope. This has now been supplied by replacing the original Munich clock of the great equatorial of the Observatory by a new one, on the principle of the spring governor, invented by the Messrs. Bond. This clock, which was made by Messrs. George and Alvan Clark of East Cambridge, carries the telescope with admirable evenness and regularity of motion.

“Immediately upon its completion, at the invitation of the Director of the Observatory, Messrs. Whipple and Black commenced a new series of experiments, and have succeeded in transferring to the plate,

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by the collodion process, images of the fixed stars to the fifth magnitude, inclusive, with singular and unexpected precision.

“ The most remarkable instances of their success are the simultaneous impressions of the group of stars composed of Mizar of the second magnitude, its companion of the fourth, and Alcor of the fifth magnitude.

“ The following measurements of the angular distance of the companion from Mizar were taken from the plates with the aid of the micrometer microscopes of the transit-circle. The distances represent the angles subtended by the images formed at the chemical focus, and measured from the optical centre of the object-glass. •

Plate I. April 27th, 1857 Distance = 14.7
VI. • 27th,"

= 14.7
II. “ 28th, 6
IV. " 30th, 66
VII. May 6th,"

= 14.5
" " 6th, ".

= 14.1 « 6th, "

= 14.3 “In consequence of a difficulty in the way of properly applying the microscope of the transit-circle, without incurring a sensible error of measurement independent of that attributable to the photographic process, the same plates (with the exception of II.) and six others were subjected to another form of micrometer, hastily arranged for the purpose, and less open to similar objection, though still sensibly imperfect. With this the following results have been obtained.

I. April 27, 1857 Dist. = 14.44 Ang. of Pos. = ... IV. “ 30, “ “ = 14.34 " V. - 30,

14.77 VI. " 27, “

14.52 VII. A. May 6,

14.34 46 B. 66 6.

14.19 I 14.44

146° 40 14.59 14.55 14.41

14.35

= 14.70 B. " 8 " = 14.77

147 55 Mean = 14.49

Mean = 147 49

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“ For the sake of comparison, we will quote from the Positiones Media of Professor Struve the following measurements of the same stars. 1755 Dist. = 13.9 Ang. of Pos. = 143.1 Obs. by Bradley. 1780 " = 14.1 " 6 = 146.8 " “ Herschel. 1820.9 " = 14.63 0 6 = 146.2 " " Struve. 1830.6 6 = 14.37 " " = 147.6* • " " 1840.8 = 14.35 "

= 147.7 " " 1847.6 6 = 14.25 1 16 = 148.2 16 6 6 The mean of Struve's Distances is = 14".40

« Positions is = 147o.4 " " Photographic Distances is = 14".49

" Positions is = 147o.8

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“The probable error of a single photographic distance is 04.12, or quite as small as that attributed by Struve to a single direct measurement. The former method has thus in its first efforts attained the limit of accuracy beyond which it is not to be expected that the latter can ever be sensibly advanced. But the photographic process holds out a much better promise.

“ The two principal sources of error by which it is affected are spots on the glass plate, or impurities in the coating in the neighbor. hood of the images, and slight departures from symmetry in their form, as yet noticed only when the plate has been exposed too long to the action of the light. The latter has been the case with most of the plates from which the above measurements have been taken, and they may in consequence be slightly affected. It is certainly to be anticipated, that, by the exercise of more care in regulating the time of exposure, the symmetry of the images can be secured. A microscopic examination will in most cases serve to distinguish accidental spots in the coating, or on the glass, from the molecules, which, by their aggregation, show the action of light.

“ The real difficulty, perhaps insurmountable, which now prevents a most extensive application of photography to astronomy, is the deficient sensitiveness of the processes in use. Unless photographs of stars as low, at least, as the eighth magnitude can be obtained, its use must be restricted to comparatively few double stars. Should, however, this impediment be overcome, and photographic impressions be obtained from stars between the sixth and eleventh magnitudes, as has already been done for those between the first and the fifth, the extension given to our present means of observation would be an advance in the science of stellar astronomy of which it would scarcely be possible to exaggerate the importance.”

Professor Bowen read an extract from a lecture of Professor Faraday, on “Conservation of Force," and remarked upon it as expressing views remarkably coincident with those offered by him in the paper read at a late meeting.

Professor William B. Rogers criticised the views in question at some length, and the subject of the nature of force was further discussed by Dr. Holmes, Dr. W. F. Channing, and Professors Bowen and Treadwell.

Dr. B. A. Gould, Jr. gave a history and description of the Calculating Engine of Babbage, and also of that of Mr. Scheutz of Stockholm, which was exhibited at the Paris Industrial Exhibition, and has since been purchased for the Albany Observatory.

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