SCIENCE. 



83 



it with the nebula with very great interest at the time, and 

 I cannot agree with Mr. Common in preferring Father 

 Secchi's drawing. It seems to me that the Earl of Rosse's 

 drawing is much the more accurate in respect of details. 

 As regards contour and outline, that depends very much 

 upon the amount of light, which impresses one man's 

 eye rather than another's so that the general outline may 

 be extended much more in one case than in another. 

 Lord Rosse's drawing does not give the whole sweep of 

 the nebula, and does not take in so extensive a field as 

 Father Secchi's drawing. Lord Rosse's drawing is bet- 

 ter seen in the black upon white print than in the white 

 upon the black ground. 



Mr. Common said that there was a great black channel 

 in the nebula, which is well shown in Father Secchi's 

 drawing, but is lost in the Earl of Rosse's drawing. The 

 latter drawing seemed to him too full of detail.* 



Mr. Ranyard said although the actual brightness of 

 various parts of an object like a nebula or corona cannot 

 be judged of from the opacity of corresponding parts of 

 photographs, yet a photograph will enable one to tell 

 with great certainty which is the brightest region of 

 the object photographed, and it affords a very valuable 

 permanent photometric scale, by which various degrees 

 of brightness of one region relatively to another may be 

 judged of. For example, Dr. Draper's photograph shows 

 that a nebulous mass on the preceding side ot the trapez- 

 ium is the brightest region ot the nebula. This does not 

 correspond wnh any of the draw ngs. It is of course 

 possible that the actinic light of the nebula does not cor- 

 respond with its luminosity as observed by the eye, but 

 this supposition is not very probable, as the spectros- 

 cope does not show any striking differences in the com- 

 position of the light of the nebula. The photograph en- 

 ables us to judge very well of the relative magnitudes of 

 the stars involved in the nebula. I have compared the 

 magnitudes of the images of the stars in the photograph 

 as enlarged by irradiation, with the magnitudes of the 

 same stars as given by Liaponnov, and 1 find that they 

 correspond very accurately. No doubt it may also be 

 assumed that the brightness of various regions of the 

 nebula may be compared with equal satety by noting the 

 opacity of corresponding parts of the photographic film. 

 With regard to Father Secchi's drawing and the drawing 

 of the Earl of Rosse, I agree with Mr. De La Rue that 

 I rather prefer the Earl of Rosse's. It shows a much 

 smaller region of the nebula, and I must remark that I 

 have not much faith in the existence of these outlying 

 nebulous structures shown in Secchi's and Tempel's 

 drawings. If such structures exist the nebula would 

 occupy an area of more than a degree, and it ought to be 

 seen with the naked eye better than with any telescope. 

 Every one is familiar with the way in which a faint struc- 

 ture like the tail of a comet — which can be easily seen 

 with the naked eye — is lost when viewed with the best of 

 telescopes. A telescope of whatever aperture will not 

 increase the brightness of an object occupying a sensible 

 area. 



Mr. De La Rue : Lord Rosse's drawing does not em- 

 brace such a large area as Secchi's, and you do not see 

 the contour definitely marked as you do in Secchi's. If 

 you cover those parts of Secchi's drawing down to the ex- 

 tent of Lord Rosse's drawing then the difference of out- 

 line that strikes Mr. Common would to a great extent 

 disappear. 



Mr. Mitchell : If you get a definite chemical com- 

 pound with which you make your photographic plate, 



* [Note by Mr. Common.] Reference to the drawings here mentioned 

 was only made incidentally, and with regard to one point. As to which 

 of the two is the better one, 1 have no doubt in my mind, nor need any one 

 have who looks at them with a rtcollection of the real object. What I 

 wanted to point out was, that owing to a proper contrast not having been 

 made in Lord Rosse's drawing, the general appearance, or what we would 

 tails the leading features, was lost, and a drawing excellent in all the de- 

 call fails in these leading features. 



and can obtain a definite exposure, and know the other 

 conditions of temperature, and so on, I think that it can 

 not be doubted that you would have a more reliable 

 record than if the varying conditions of the brain, at one 

 time and another, have to be taken into account. If the 

 condition of one man's brain has to be compared with 

 the condition of the brain of another man, physiological 

 difficulties come in which may be avoided by means of 

 photography. In comparing photographs you have only 

 mechanical differences and physical conditions to con- 

 sider, which certainly involve much less complication 

 than physiological differences. 



ASTRONOMY. 



MAGNITUDE OF JUPITER'S THIRD SATELLITE. 



On the evening of February 2, Jupiter was passing- 

 near the star B. A. C. 303 (73 Piscium, and the opportu- 

 nity was taken at the Observatory of Harvard College to 

 compare photometrically the third satellite of the planet, 

 with the star. Three observers took part in the work, 

 and four sets of measurements, each consisting of eight 

 single comparisons, were made. The result obtained was 

 that the star was fainter than the satellite by 0.38 magni- 

 tudes of Pogson's logarithmic scale. For the magnitude 

 of the star we have 6.16 by the mean of the available es- 

 timates on record, and 6.17 by the observations made at 

 this observatory with the meridian photometer. The re- 

 sulting magnitude of the satellite is 5.28 or 5.29, in close 

 agreement with the value, 5.24, found by a very different 

 method, in the Annals ot the Observatory, Vol. XL, p. 

 276. 



Swift's Comet. — We are indebted to Prof. Pickering 

 for the following list of dates on which observations of 

 Swift's Comet (1880 e), were obtained at Harvard College 

 Observatory, by Mr. Wendell : 



1880, Nov. 3, 1880, Nov. 27, 1880, Dec. 28, 



" 8, " 29, " 30, 



" 9, Dec. 2, " 31, 



11, " 3, 1881, Jan. 1, 



" 18, " 4. " 3. 



" 19. " 7. '' 7. 



21, " 11, " 8, 



22, " 19, " 18, 

 " 23, " 22, " 20. 

 "26. " 23. 



Urania. — The first number of the new International 

 journal of Astronomy contains in a very convenient 

 lorm of 24 demy 4to pages, a number of interesting arti- 

 cles. Among others are the following papers : " Obser- 

 vations of the Spectrum of Comtt 1880 d, (Hartwig) at 

 Dunecht," by Copeland and Lohse. " A New Planetary 

 Nebula," by Dr. Copeland. " Observations of Comets 

 1880 b, c, and d, at Dunecht. " Uber die Auflosung der 

 Lambert'schen Gleichung fur Parabolische Bahnen, by 

 Professor Klinkerfues. 



Prof. William A. Rogers, of Cambridge, has re- 

 cently made a visit to Washington to compare the copies 

 of the English and French standards of length, with the 

 standards of our Government deposited at the Coast Sur- 

 vey Office. Prof. Rogers obtained very accurate copies of 

 the yard and metre during January and February, 1880 

 having made a trip to Paris and London for that pur- 

 pose. 



We learn of the recent death of Baron Dembowski, 

 the well-known double-star observer, at the age of 69. 

 For upward of twenty-five years he had devoted himself 

 to the re-measurement of the stars of the Dorpat Cata- 

 logue, and foi this work was awarded in 1878 the gold 

 medal of the Royal Astronomical Society. 



w. c. w. 



