14 



NATURE 



{May 7, 1874 



more so in 1761. Such observations were then wanting. 

 Hence the application of this method was not successful, 

 and results of that transit were unsatisfactory. 



Not daunted by the comparative failure of that attempt, 

 the astronomers of last century made vigorous efforts to 

 make the transit of 1761 successful. The transit of 1761 

 was utilised in so far as it pointed out the difficulties in 

 this kind of observation and gave them an approximate 

 value of the sun's parallax to help them in choosing the 

 most advantageous stations from which to observe the 

 next transit. 



H alley had no conception, when he proposed this kind 

 of observation, of the difficulties attending it. The diffi- 

 culty chiefly consists in determining accurately the exact 

 instant when the contact seems to take place. The values 

 which have been deduced from the observations of last 

 century, and especially of the year 1761, have varied con- 

 siderably according to the mode of reducing the observa- 

 tions. Thus in 1761 Lalande * found, from the observa- 

 tions of Pingrc?, g"'4 for the solar parallax, while 

 Maskelyne found from the work of Mason and Dixon 

 S"-6 ; Short t made it 8'-65 ; Wargentin, 8"-l to 8"'3. 

 Encke t showed that the differences were partly due to 

 an error in the longitude of Rodriguez. This question 

 will be capable of further discussion after this year, as 

 Rodriguez is one of the stations chosen by the English 

 from which to observe the coming transit. 



Since the observers are likely to differ considerably in 

 the manner in which they observe the contact, and since 

 it is difficult for us to be sure that all observers have really 

 actually noted the same phenomenon, photography is 

 once more brought to our aid. Sometime .ago M. Janssen 

 proposed a method for determining by the aid of photo- 

 graphy the exact instant of contact. The value of his 

 method was immediately recognised, and steps have been 

 taken to utilise it. The method consists essentially in 

 exposing different parts of a prepared photographic plate 

 in succession to the sun's light, so as to photograph that 

 portion of the sun's limb at which the planet is visible. 

 By the aid of no very complicated mechanism a circular 

 plate is so arranged that sixty different portions of its 

 surface near the circumference are successively brought 

 into position, and exposed to the action of the sun's rays. 

 The plate completes a revolution once in a minute, so that 

 sixty photographs are taken at intervals of one second. 

 A person who is observing with a telescope can easily 

 give a signal to commence these photographic operations 

 at the proper time. Thus one of the photographs will be 

 sure to give us an indication of the time of true contact. 

 Furthermore each one of the photographs taken at one 

 station can be compared with a corresponding one taken 

 at another station, so as to give us a means of deducing 

 the sun's paralla.x. The advantages of this method are 

 enormous. The uncertainty which exists with respect to 

 eye observations is in a great measure due to fluctuations 

 arising from tremors in the instruments, and variations in 

 the density of the intervening air. In the photographic 

 method, means have been taken to avoid these tremors 

 as far as possible ; and the instantaneous manner in 

 which the photographs are taken will reduce these un- 

 certainties to a minimum. 



Various suggestions have been made as to the possi- 

 bihty of observing the exact time of the external contact 

 by using a spectroscope in a beautiful manner originally 

 executed by Mr. Lockyer and M. Janssen for observing 

 the solar protuberances. Father Secchi has, in a very 

 able memoir, pointed out a way by means of which this 

 can be done ; M. Zollner has likewise pointed out the 

 advantages of this meihod. 



The observation of external contact is doubtless very 

 useful as supplementary to the internal contact. The 

 chief difficulty consists in the uncertainty of fixing the 



• Phil. Trans., vol. lii., p. 647. 



t Ibid., p 648. 



X Zach. Cortesp. ii., 1810, p. 367- 



telescope in the proper position, so as catch the exact 

 point of the sun's limb. This difficultv would certainly 

 be to a large extent obviated by the employment of the 

 ingenious adjustible ring-slit devised by Mr. Lockyer. 

 This device has, we believe, been fully tested, with satis- 

 factory results. It is much to be regretted that more 

 observations to test its utility have not been made ; as 

 on this account it is not likely to be employed in the 

 coming transit. 



We have now completed the geometrical examination 

 of the nature of the observations on the transit of Venus, 

 by means of which the sun's parallax will be deduced. 

 The complete examination of the question, including 

 analytical methods, cannot be here dwelt upon. Anyone 

 who is interested in this should consult the valuable work 

 " Les passages de Vdnus sur le disque Solaire," by M. 

 Edmond du Bois, lately published, in which the theore- 

 tical part of the question is very fully investigated. 



Recapitulation. — Before leaving the technical view 

 of the matter it will be well to recapitulate what has 

 hitherto been stated. 



1. We know the relative dimensions of the solar system 

 accurately ; but we do not know the scale. 



2. The determination of the distance of the earth from 

 the sun or from any of the planets, at a fixed date, fixes 

 the scale. 



3. This may be determined (i) by the aid of a transit 

 of Venus ; (2) by an opposition of Mars ; (3) by a know- 

 ledge of the velocity of light combined with observations 

 of echpses of Jupiter's satellites ; (4) by the velocity of 

 light and the constant of aberration ; (5) by the calculated 

 effects of the sun's disturbance upon the lunar motions. 



4. A transit of Venus may be utilised : — 



(a) By the determination of times of contact at dif- 

 ferent stations, combined with a knowledge 

 of the longitudes of these stations. 



0) By determining the least distance between the 

 centres of the sun and Venus during the 

 transit, observed from different stations. 



5. This last determination may be made by either of 

 these methods : — 



(i) The Photographic Method. 



(2) The Hcliometric Method. 



(3) The Method of Durations. 



George Forbe.s 



NOTES 



The Bo.ird of Trinity College, Dublin, have appointed R. Ball, 

 LL.D., F.R.S., to be Royal Astronomer of Ireland, on the 

 foundation of Dr. Andrews. The announcement of this appoint- 

 ment will be received with every satisfaction, as Dr. Ball has 

 ah-eady, while acting as assistant to Lord Rosse, distinguished 

 himself as a practical observer. We feel sure he will not forget 

 to profit, or omit to allow astronomical science to profit too, by 

 the excellently appointed observatory at his command. This 

 vacates the chair of Applied Mathematics in the Royal College 

 of Science, Dublin. 



At a meeting of the donors of the Yorkshire College of 

 Science (see N.vture, vol. ix. p. 157) held at Leeds last Thurs- 

 day, the constitution of the College was agreed upon, and a board 

 of governors elected. The sum required to establish a College 

 of Science in any way worthy of Yorkshire would be 6o,oco/., 

 of which only about 25,000/. has as yet been collected. With 

 t his sum, however, we are glad to see that it has been resolved tp 

 make a start, and we have no doubt that when the practical 

 benefits of the institution become evident there will be little 

 difficulty from lack of funds." We trust with Lord F. Caven- 

 dish that, ere long, the institution just organised will occupy in 

 Yorkshire a position similar to that occupied by Owens College 

 in Lancashire. Several speakers referred to the fact that in the 



