yune 29, 1876] 



NATURE 



J93 



THE SATELLITE OF VENUS ^ 



A N indirect result of the recent Transit of Venus has 

 -^"^ been the revival of a nearly forgotten but not unin- 

 teresting speculation as to the possible existence of a 

 satellite accompanying her. Nothing of the kind was 

 observed on the late occasion, but the planet's path was 

 so far from central that an attendant might readily have 

 remained outside the solar disc ; and therefore, though 

 the negative evidence, if it had required additional 

 strength, would have received it from this non-appear- 

 ance, it would not have been rendered absolutely conclu- 

 sive on that ground alone ; and, so far as the Transit is 

 concerned, there is still room for an essay like that 

 before us, which, previous in composition though subse- 

 quently published, advocates the affirmative opinion. 

 That opinion, after so many years of additional observa- 

 tion since Lambert's memoir in 1777, is not likely to 

 find favour with astronomers now, and certainly will not 

 be established by the present treatise. It is an unplea- 

 sant task to express any other than a favourable estimate 

 of any work undertaken with a view to enlarge the 

 boundary of knowledge ; but in the present instance it is 

 unavoidable. There is, indeed, a very considerable 

 accumulation of historical matter, and there are some 

 pleasant anecdotes, and a few valuable and little known 

 facts ; but the materials of some portions at least are 

 neither complete nor accurate ; the pretermission of 

 recent discoveries — especially spectroscopic — is some- 

 times simply unaccountable ; and the hypotheses occa- 

 sionally partake of an extravagance that outstrips all 

 probability. The subject is, however, as has been 

 remarked, not devoid of some interest, nor, to say the 

 truth, disentangled from some perplexity of an obstinate 

 character ; and it is worthy of a more satisfactory eluci- 

 dation, which might be comprised in a narrow compass, 

 as its literature is not extensive. A few remarks only 

 can be attempted here. 



That something strongly resembling a satellite has 

 been occasionally seen near Venus, especially about the 

 middle of the last century, is beyond a doubt. It is 

 equally certain, and familiar to all experienced observers, 

 that reflected images, or technically " ghosts," may, under 

 certain circumstances, be formed in the eye-piece of the 

 telescope, and might be the means of causing deception : 

 and the whole matter is reduced to the simple inquiry, 

 whether all the recorded instances admit of this easy 

 explanation ; though, if they do not, it must be remem- 

 bered that the existence of a satellite would not neces- 

 sarily follow. 



The Abbot Hell, who published an elaborate disserta- 

 tion on the subject in the appendix to the Vienna Astro- 

 nomical Ephemerides for 1 766, seems to have been the 

 first to study systematically the formation of telescopic 

 ghosts. The Vienna Observatory was possessed in those 

 days of two good EngHsh telescopes, left to it in 1757 by 

 Cardinal de Trautson ; a 2 ft. Gregorian, and a 4^ ft. 

 Newtonian. About December in that year, the Abbot, 

 examining Venus with the former instrument and a 

 power of 70 or 80, perceived a star of an ill- defined 

 aspect near it like a little comet, but as it was in- 

 visible both in that Newtonian and in another of 

 the same construction of 4 ft., he referred it to a re- 

 flection from the interior of the tube. In March 1758, 

 Venus being at her greatest elongation, the illusion 

 returned, on which he blackened the tube, and for some 

 days did not see it again ; but when at length it re- 

 appeared, on moving his eye very gently towards the eye- 

 piece he found it change into a perfect image of a 

 satellite with the phase of the primary. Beyond the limit 

 of barely hallf a line either way from this position, it was 

 invisible. When Venus occupied the centre of the field, 



' " Der Venusmond," &c., von Dr. F. Schorr. Brauasckwiei^, 1375, 

 Pf>. 186. 



this " spectrum," as he calls it, was near the edge ; as he 

 moved his eye round, or up and down, the image moved 

 the same way, generally disappearing in the neighbour- 

 hood of the planet. A set of experiments instituted in 

 consequence satisfied him that this image was formed by 

 rays reflected first from the convexity of the " pupil " 

 (cornea), and a second time from the concave face of the 

 meniscus lens which in this case formed the eye-glass, 

 though it would be shown by any eye-piece possessing a 

 surface concave towards the retina. Cases were even 

 possible, but difficult in management, when an image 

 might be seen, though the object was not in the field ; 

 but this was formed by rays passing outside the telescope, 

 and the ghost would be inverted and of much smaller 

 dimensions. The magnitude of the image would depend 

 on the proportion of curvature of the reflecting surfaces. 

 This being once understood, the Abbot found that he 

 could always produce, for himself or others, a spurious 

 satellite of Venus, or Mars, or Jupiter, under the followmg 

 essential conditions : — That the power should not be less 

 than 50 or 80, or the image would be too minute to be 

 visible, or would only resemble a small star ; — that the 

 eye must be placed at a definite distance from the eye- 

 glass, and be moved most deliberately and cautiously 

 backwards and forwards to find that point, the limit of 

 visibility being sometimes only a quarter of a line either 

 way ; — and that the eye must be a little on one side of 

 the optical axis, or the image will coincide with its 

 primary. And it becomes readily intelligible why an 

 observer, ignorant of these conditions, may never be able 

 to recover an image which he had once accidentally seen. 

 Thus far, in substance, the astronomer of Vienna, who 

 certainly deserves credit for his ingenious and careful 

 investigation. His reasoning is, nevertheless, a curious 

 and instructive exemplification of the way in which a pre- 

 conceived opinion may block up the mental view, and 

 prevent a sound argument from being carried out to its 

 legitimate consequences. 



We are now in a position to examine how far this 

 criterion is applicable to the recorded phenomena. Of 

 these. Dr. Schorr has enumerated sixteen, in a table 

 taken apparently from Lambert, but with the additioa of 

 an observation by Andfeas Meier (Mayer). Hell had 

 given three from Fontana, but Lambert seems to have 

 thought one only of any consequence, and even this may 

 well be omitted, leaving the following for our considera- 

 tion. 



The name of Cassini at the head of them at once com- 

 mands attention, but there is nothing in his two obser- 

 vations in 1672 and 1686 that does not lend itself to 

 Father Hell's hypothesis, excepting the care and expe- 

 rience of such an observer, who must have been familiar 

 with every telescopic defect. The observation of Meier, 

 which seems to have lain unnoticed in the Astron. Jahr- 

 buck, 1788, till brought forward by Schorr, is on that 

 account worthy of being cited in full " 1759, May 20, 

 about 8h. 45m. 50s., I saw above Venus a little globe ot 

 far inferior brightness, about i^ diam. of Venus from her- 

 self. Future observations will show whether this little 

 globe was an optical appearance or the satellite of Venus. 

 The observation was made with a Gregorian telescope of 

 thirty inches focus. It continued for half an hour, and the 

 position of the little globe with regard to Venus re- 

 mained the same, although the direction of the telescope 

 had been changed." During so lengthened an obser- 

 vation it seems natural to suppose that the eye must have 

 been repeatedly removed and replaced, which could not 

 have occurred without the detection of an optical illusion. 



In 1761, when the expected transit drew attention to 

 Venus, Montaigne, at Limoges, was persuaded to under- 

 take the inquiry, though he had little faith in the existence 

 of the satellite, and was not greatly disposed to enter 

 upon an examination in which so many great men had 

 failed. However, on May 3 he saw a small cresce.it 



