June I, 1876] 



NATURE 



91 



theless, he is known to have his pet theories about the antiquity 

 of the Moa, and is very impatient of any contradiction. 



I have thought it right to offer this explanation in order to 

 prevent your readers being misled on a subject of considerable 

 scientific interest. Julius von Haast 



Canterbury Museum, Christchurch, N.Z., March 14 



Visibility of the Satellites of Uranus 



The question of the visibility of these satellites in telescopes 

 of moderate dimensions has lately excited considerable attention, 

 but it does not appear that this question can be settled by any 

 amount of verbal discussion. I take the liberty, therefore, to 

 propose two test objects by means of which any one can, I think, 

 satisfy himself whether he can see these satellites or not. 



1. The companion of Regulus, north, preceding, and distant 

 about three minutes of arc, has itself a small companion, which 

 was discovered by the late Prof. Winlock. Any one who can 

 see this small companion may be certain that he can observe the 

 two outer satellites of Uranus and the satellite of Neptune. 



2. The star of fifth magnitude, A Leonis, has a companion 

 discovered at the Naval Observatory by Mr. G. Anderson. Any 

 one who can observe this companion can, I think, see the two 

 inner satellites of Uranus when at their elongations. 



Of course in the case of such faint objects very much depends 

 on the condition of the atmosphere, but the above tests are very 

 nearly correct. Asaph Hall 



Washington, May 14 



Protective Resemblance in the Sloths 



In a note upon the above subject, dated December 29, 1875, 

 which appeared in vol. xiii. p 187 of Nature, I omitted to quote 

 a passage from a letter wiitten by Dr. Berthold Seemann to the 

 late Dr. J. E. Gray (rated April i, 1871), with regard to a speci- 

 men oi Arctopithrcus, of a well-marked green colour, obtamed by 

 the former naturalist in Nicaragua. Of this Sloth he i-ays, inter 

 alia : — " It should be borne in mind that it has almost exactly 

 the same greyish-green colour as Tillandsia usneotdts, the so- 

 called 'vegetable horsehair' common in the district; and if it 

 could be shown that it frequented trees covered with that plant 

 (a point I hope to ascertain during my next visit in June next), 

 there would be a curious case of mimicry between this Sloth's 

 hair and the Tillandsia, and a good reason why so few of these 

 sloths are seen." (Note on the species of Bradypodtda in the 

 British Museum, by Dr. J. E. Gray, F.R.S., Proc. Zool. Soc, 

 May 2, 1871.) It would be interesting to know whether Dr. 

 Seeman succeeded in solving this question ; I am, however, not 

 awaie of any later reference made by him to this subject. 



I here take the opportunity of correcting two misprints in 

 my former letter, both of them in the Latin quotations, viz., 

 "cum" for "eum," after the woid "velleri," in the first, and 

 " coque " instead of "eoque" after the woid "possint," near 

 the tnd of the second passage. J. C. Galton 



OUR ASTRONOMICAL COLUMN 



The Secondary Light of Venus. — During the next 

 few weeks a very favourable opportunity will be afforded 

 to observers in these latitudes for further examination of 

 the planet Venus, with the view to a satisfactory solution of 

 what must yet be regarded as a questio vexala—ihe 

 visibility of that part of the disc, which is unillumined by 

 the sun, as the planet approaches or recedes from the 

 inferior conjunction. 



The subject is treated in detail in a communication to 

 the Bohemian Acadeiny of Sciences, from Prof. Safarik 

 of Prague, entitled " t)ber die Sichtbarkeit der dunklen 

 Halbkugel des Planeien Venus, " which appears in Stlz- 

 uns;sberichte der k. bohmischen Gesellschaft der Wissen- 

 scha/len, July 18, 1873. The author has collected to- 

 gether the many scattered observations extending over 

 upwards of one hundred and fifty years, and presents also 

 an outline of the various explanations which have been 

 put forward. 



The earliest mention of the faint illumination of the 

 dark side of Venus is by Derham, in a passage in his 

 Astro-Theology, to which attention was first directed by 

 Arago. Derham refers to the visibility of the obscure 

 part of the globe " by the aid of a light of a somewhat 

 dull and ruddy colour." The observation is not dated, 

 but appears to have been prior to the year 1714. A 

 friend of Dcrham's is also stated to have perceived the 

 same illumination very distinctly. 



The next observations are by Christfried Kirch, second 

 astronomer of the Berlin Academy of Sciences, June 7, 

 1 72 1, and March 8, 1726, and were found in his original 

 papers and printed in Ast. Nach. No. 1586. The image 

 on the first occasion was tremulous, but though he could 

 hardly credit his vision, he appeared to discern the dark 

 side of the planet. In 1726 he remarked that the dark 

 periphery seemed to belong to a smaller circle than the 

 illuminated one. Kirch observed with telescopes of six- 

 teen and twenty-six feet focal length, powers 80 and 100. 

 Two other persons confirmed his observation in 1726. 



The next observation in order of date, was found by 

 Gibers, in " Observationes Veneris Grypiswaldensis," cited 

 by Schroter in his observations of the great comet of 

 1807. It was made by Andreas Mayer, Professor of 

 Mathematics at (ireiswald : on October 20, 1759, he 

 observed the meridian passage of the planet, then at a 

 south declination of 21^°, with a six-foot transit instru- 

 ment by Bird, power not much over 50, and has the 

 remark— " E^tsi pars lucida Veneris tenuis admodum erat, 

 nihilominus integer discus apparuit, instar lunae crescentis 

 quae acceptum a terra lumen reflectit." As Prof. Safarik 

 justly observes, considering the circumstances under which 

 Mayer's observation was made with the planet only 10" 

 from the sun, and not more than 14° above the horizon, 

 the phenomenon on this occasion must have had a most 

 unusual intensity. 



It does not appear that Sir W. Herschel at any tirre 

 perceived the secondary light of Venus, though he remarked 

 the extension of the horns beyond a semi-circle. 



Von Hahn, at Remplin, in Mecklenberg, the possessor 

 of excellent telescopes by Dollond and Herschel, was for- 

 tunate in viewing the dark side of Venus on frequent occa- 

 sions during the spring and summer of the year 1793, and 

 he is considered by Safarik to have witnessed the illumi- 

 nation of this part of the disk under more varying con- 

 ditions than any other observer. The light is described 

 as grey verging upon brown. Von Hahn's observations 

 were made with various instruments and at different 

 hours of the day. 



Schroter, at Lilienthal, on several occasions between 

 the years 1784 and 1795, ^^^ remarked in full sun- 

 shine the extension of the horns of the crescent many 

 degrees beyond the semicircle, the borders of the dark 

 hemisphere being faintly illuminated with a dusky grey 

 light; but on February 14, 1806, at 7 P.M., he saw 

 for the first time the whole of the dark side, as he ex- 

 pressed it, " in ausserst mattem dunkeln Lichte." The 

 sharply-defined contour had an ash-coloured light ; the 

 surface was more dimly illuminated. Schroter, in 

 recording this observation, expresses his surprise that 

 during the many years he had observed the planet, part 

 of the time with his 27-feet reflector, with the full aper- 

 ture of 20 inches, he had not previously perceived the 

 whole of the dark side, but he was satisfied there was no 

 illusion. At this time one-eighth of the diameter of 

 Venus, about 48", was fully illuminated, the planet casting 

 a very sensible shadow. 



Harding, observing at Gottingen on January 24 of the 

 same year, with a lo-feet Herschelian reflector, power 84, 

 and full aperture of 9 inches, saw the whole dark side of 

 Venus shining with a pale ash-coloured light, very dis- 

 tinctly perceived against the dark ground of the sky. The 

 appearance was too evident to allow of the suspicion of 

 an illusion ; it was the same in all parts of the field of 



