28 



NA TURE 



[May 14, 1874 



distance of the sun from the earth is about 95 millions 

 of miles. It now appears that the true distance is some- 

 where about gii millions of miles. The annexed table 

 gives the values of the sun's parallax and distance as 

 determined by different methods. 



The uncertainty of observation which Mr. Stone aimed 

 at clearing away is one of a very curious optical character. 

 It is found that Venus at the time when she has almost 

 completely entered within the sun's dirc does not retain 

 her round aspect, but becomes pear-shaped, or at least 

 connected with the sun's limb by a "black drop " or 

 " ligament." This lig.Tment sometimes appears simply 

 as a fine black thread connecting the planet with the limb 

 of the sun. One observer in 1769 saw a number of black 

 cones shooting out to the sun's edge in a fluctuating 

 manner. 



Many of these phenomena were doubtless due to bad 

 definition of the telescope employed, or to the instability 

 of its mounting. But the existence of a " black drop " 

 even under the most favourable circumstances cannot be 

 doubted ; it was well observed in the case of a transit of 

 Mercury that occurred in 1 868. ft If the planet be entering 

 upon the solar disc, the first phase occurs when the edges 

 of the sun and planet st'ein to be in contact. The second 

 phase occurs at the instant when the " black drop "breaks 

 off and a flood of light sweeps in between the pl.inet and 

 the sun. This occurs very suddenly, and has been sup- 

 posed to indicate the true time of actual contact. 



By referring to the Philosophical Transactioiis of 1769- 

 70, a large number of descripiions of the phenomenon 

 may be read. Some of the appearances are shown in 

 Fig. 14, they are copied from the originds by Bevis, 

 Hirst, I5ayley, and Mayer, respectively — Prof. Grant states 

 that the last one bears a resemblance to the appearance 

 of Mercury as seen during its transit in 1S68 from the 

 Glasgow Observatory, the sun being near the horizon. 



In the case of that transit of Mercury, studied by six 

 experienced observers at Greenwich Observatory, two 

 curious facts appear. Firstly, the times of contact as 

 determined by ditlerent observers vary to the extent of 

 I3i seconds. And secondly, the shape of the planet 

 varied considerably with different observers. 



Mr. Stone having noticed a confusion in the language 

 of the astronomers of the last century as to which of the 

 two phases was observed, carefully re-studied their words; 

 and by supposing the two phases to be separated by a 

 constant interval of time, he utilised both kinds of obser- 

 vation. This constant interval of time was deduced from 

 all the observations, and found to be about 17 seconds. 

 In this manner he arrived at the more accurate value of 

 the sun's parallax. 



It has been asserted that astronomers claim undue 

 credit for the accuracy of their measurements, since 

 Encke made an error of three or four millions of miles 

 in the calculation ol the sun's distance. This is not so. 

 A chemist may be able to weigh many substances with 



• Monthly Notices, xxviii 

 J Ibid, xxiv., 8 

 II Cotnptes Rendus, July ; 

 •" Ibid, 1873. p. 34". 



+ Ibid, xxiii , 1B3. 

 § Ibid, xxvii., 271. 

 H Ibid. 1862. p. 502 

 tt Monthly Notices 



an error of iJo per cent, or less ; but if the substance to be 



weighed be only yjjj of a milligramme, he might have 

 a larger percentage error. When we consider how ex- 

 tremely small an angle the solar parallax is, it is asto- 

 nishing to find so great a concordance between the results 

 of different methods. 



As to the cause of the phenomenon of the " black drop," 

 Lalande ascribed it to irradiation. Irradiation is that 

 curious phenomenon in virtue of which a star, or any 

 bright object, appears larger than it really is. If a thin 

 platinum wire be intensely heated by the passage of an 

 electric current, it seems, to a person distant about fifty 

 feet, to be as thick as a pencil. In this way the sun's 

 diameter seems to be increased. The sun's light also 

 encroaches upon the disc of the planet and makes it seem 

 to be smaller than it really is. But when Venus and the 

 sun have their edges almost in contact, as shown by the 

 dotted line in Fig. 15, then there is no light at that point 

 which can encroach ; hence we see at this point the 

 " black drop " to which allusion has been made. 



Father Hell, one of the observers in 1769, ascribed the 

 phenomenon of the "black drop" to the sensible size 

 v/hich an illuminated surface must have before it can be 

 visible. There is probably some truth in each of these 

 suppositions. 



As to the cause of irradiation, it is difficult to speak 

 with certainty. It is probably caused in part by the 

 telescope and in part by the eye. Great confusion has 

 been introduced by persons neglecting to separate two 

 perfectly distinct phenomena. True irradiation is only 

 observed with a powerful light. With less illumination 

 similar results may be seen, but they are of a different 

 nature, and are produced between the foimation of an 

 image on the retina and its reception by the brain. In 

 accordance with the customary nomenclature, this error 

 of vision may be called the iiienlal aberration of the eye. 

 It is a perfectly definite phenomenon capable of accurate 

 invesiigaticn, and M. Plateau has made measurements of 

 the mental aberration of his own and his friends' eyes.* 

 True irradiation may be caused either wholly or in 

 part, by the spherical aberration or the chromatic aber- 

 ration of the eye, or by diffraction, or by a spreading of 

 the excitement of the nerves of the retina, which gives 

 rise to the sensation of vision over a sensible space. In 

 a telescope it is probably chiefly due to diffraction. 



The success or failure of all observations of contact in 

 the coming transit will to a great extent depend upon our 

 knowledge of the nature of this appearance. For this 

 reason numerous experiments have been made with the 

 object of gaining information upon the question. The 

 Russians, Germans, Americans, and English have all 

 mounted artificial transits of Venus for the practice 

 of observers. The arrangement adopted by the As- 

 tronomer Royal consists essentially of a metal disc 

 with two arcs of circles drawn upon it to represent 

 the sun's edge with the metal between them cut away. 

 Behind these there passes a glass plate with a circle of 

 metal to represent Venus let into it flush with its surface. 

 The glass plate is moved by clock-work so that the 

 different phenomena are observed in succession exactly 

 as they will be seen in the true transit. As the artificial 

 planet passes in succession the two arcs representing the 

 sun's edge, the phenomena of ingress and egress are succes- 

 sively observed. Before contact takes place, the sun has 

 two cusps at the point of contact where Venus is touching 

 the edge of the sun. The distance between the points 

 of these cusps rapidly diminishes, the space between 

 them being intensely black. They suddenly meet. But 

 between the planet and the sun's edge a light shade is 

 still seen which lasts several seconds before the planet 

 appears completely detached. If instead of watching the 

 meeting of tlie cusps, the part between them be studied, 

 a sudden diminution of intensity of the blackness is seen 



• Nouv. M6m. de I'Acad. Royal de Bruxelles, t. xi. p. i, Re. 



