July 20, 1882] 



NATURE 



269 



THE TRANSIT OF VENUS 

 V\7E have received the following list of selected stations for the 

 observation of the coming transit of Venus, with the 

 observers appointed to each, according to the arrangements, so 

 far definitive, made hy the executive committee of the Royal 

 Society acting under the authority of the Treasury, and also the 

 instructions prepared for the guidance of the observers. 



For Retarded Ingress and Accelerated Egress 

 Jamaica — Dr. Copeland, Ca-pt. Mackinlay, R.A., Mr. Maxwell 



Hall. 

 Barbados — Mr. Talmage, Lieut. Thomson, R.A. 

 Bermuda — Mr. J. Plummer, Lieut. Neate, K.N. 



(The Canadian Government will have three observers with 

 six-inch instruments, besides others with smaller telescopes. 

 These observers are acting in direct concert with the British 

 Expeditions). 



Accelerated Ingress 

 Cape Observatory — Mr. David Gill (H.M. Astronomer), Mr. 



Maclear, 2ml assistant. 

 Montague Read, Cafe Colony — Mr. W. H. Finlay (1st assistant 



at the Cape), Mr. Fett. 

 Aberdeen Road, Cape Colony — Mr. Burton, Mr. C. M. Stevens. 

 Madagascar — Father Perry, Father Sidgreave--, Mr. Carlisle. 

 Durban, Natal — A telescope has been provided by the colonists. 

 Mauritius— Mr. Meld rum . 



Retarded Egress 

 New Zealand — Lieut. -Col. Tupman, R.M.A., Lieut. Coke, R.N. 

 Brisbane — Capt. Morris, R.E., Lieut. Darwin, K.E.,Mr. Peek. 

 Melbourne — Mr. Ellery and staff. 

 Sydney — Mr. Russell and staff. 



Instructions to Observers 

 The " Instructions issued by the International Conference on 

 the Transit of Venus, 1882," are recommended 6 

 adoptii n. These instructions are, in a great measure, founded 

 upon " Suggestions fcra Draft of Instructions for tbe Observers," 

 which the English Committee circulated for consideration in 

 October, 1881. But as copies of the Instructions, issued by the 

 International Conference, may not reach ol servers in the 

 English colonies who may be willing and al le to co-operate 

 effectively, and as there are some additional explain 

 cautions which the English Committee consider it desirable to 

 give their observers, the following instructions have been issued. 

 It is most earnestly hoped that all observers who are able to co- 

 operate effectively in the observation of contacts will, whatever 

 else they may observe, observe the contacts as defined ; and that 

 they -.ill emploj agnifying powers on their tele- 



scopes and use a field of view- of moderate brightness. These are 

 essentials, if their ol servations are to be combined with the 

 results of the English expeditions. 



1. It is most important that the apertures of the refractors 

 used for the observations of the contacts should be nearly equal. 

 The apertures of the telescopes available for the English expe- 

 ditions are not generally larger than six inches ; and it is 

 therefore recommended that observers with larger apertures 

 should stop them down to six inches, but not reduce them below 

 that size. It is considered that perfect instruments of four 

 inches aperture may give results sufficiently comparable with 

 those made with the six-inch telescopes to allow all the obser- 

 vations to be combined in one common discussion ; and it is 

 hoped that observations with such instruments wdl be made. 

 Observations with good instruments of smaller apertures than 

 four inches will be exceedingly valuable for combination with 

 others made with instruments of the same class, provided that 

 powers of 100 to 150 can be employed on them. Observations 

 of contact made with very low powers are useless for the objects 

 in view. When reflectors are used, apertures should not be 

 less than seven inches. 



2. The observers are requested to furnish tests of the optical 

 performances of their telescopes, such as — 



(1) The appearances presented by the disc of a bright star 

 when the eye-piece is pushed within and pulled without the 

 focus. 



(2) The power of the telescope to separate some well-known 

 close double stars. 



(3) Whether the observer was able to see the "rice grains" 

 or " granulations " on the sun's disc on the day of the transit 

 and if not, whether he is generally able to see them with the 



same instrument and a power of 150 on days of good definition. 



3. The observers are recommended to employ a first-surface 

 reflecting prism to diminish the sun's heat and danger to the 

 observer's eye, and a compensated neutral-tint wedge between the 

 eyepiece and the eye. 



4. The eye-pieces recommended are the " negative," or a 

 " Steinheil's simple achromatic positive eye-piece." When the 

 latter is used, two pairs of very fine parallel wires should be 

 placed on opposite sides of the field of view at distances corre- 

 sponding to a second of arc apart. Such wires are useful in 

 judging of the brightness of the field of view, and of the suffi- 

 ciency of the optical power employed to subdivide a second of 

 arc into tenths, and in estimating the angular separation of the 

 limbs in descriptions of any phenomena which may be seen near 

 the contact. 



5. All attempts to observe the contacts with double-image eye- 

 pieces are to be avoided. 



6. The use of Dawes' solar eye-piece is not recommended. 

 This eye-piece is exceedingly valuable for the examination of 

 small detached portions of the solar disc ; but the field of view- 

 is very limited, and, if clouds were passing, there would be 

 practical difficulties in keeping ihe point of contact a 



the centre of the field, whilst the effects of the stop would 

 certainly he injurious to the contact observations near the edge 

 of the field. 



7. The magnifying power should be about 150, and, even 

 if the definition is not good, this power should not be much 

 reduced. It is es ential that the observer should be 

 subdivide a second of arc into tenths, and to do this a hi^h 

 power is necessary. 



8. It is extremely desirable that all the observal 

 contact should be made in fields of view of nearly the same 

 brightness, and the illumination should neither 1 e 

 extreme b ich greatly complicates the ph 



seen near the contacts from the increased effects of in 

 nor one so faint that difficulties may be experienced in dis- 

 _■ changes in the illumination of the sun's limb near 

 ;ch can only bo 

 If pairs of very fine spider webs are placed at a distance corre- 

 sponding t ) a seci nd of arc in the focus of a positive eye-piece, 

 they may 1 e u-ed for the detern.inationof a suitable illumination 

 of the field of view 1 y observing at what part of the neutral-tint 

 wedge the wires can just be seen with sufficient distinctness to 

 allow the observer, without difficulty, to sub divide the interval 

 between them into tenths; but in the application of the test 

 the wedge must be shifted quickly, and the eye not allowed to 

 strain itself in attempting to separate tbe wires, or a too dull 

 field of view will be secured. The test is a delica: 

 apply, and it is probably better to replace it in practice by the 

 following: — When the sun is free from clouds, detci 

 what parts Mint wedge the sun's limb ca 



observed with comfort, and with what part of the v. \ 

 limb can just be distinctly and clearly seen. A mean portion 

 of the wedge between these extremes should be adopted as a 

 standard field of brightness with a clear sun ; and this degree 

 of brightness, learnt by practice as a habit of the eye, should be 

 adopted for the brightness of the field of view in the observation 

 of the contact-. 



9. The expressions "contact," "apparent contact," "actual 

 contact," "real contact," and "true contact," appended to 

 time records without any explanation of the sense in which 

 the word "contact" is used, are liable to be misunderstood. 

 Such expressions should not, therefore, be employed, unless a 

 description of the particular kind of contact referred to is 

 appended. 



to. The phenomena seen, by most observers, near the time of 

 contact, in a moderately bright field of view, are of a complex 

 character, and extend over considerable intervals of time. It 

 is therefore necessary to direct the attention of the observers 

 to some distinctive phases which all those, who have sufficient 

 optical means, should certainly see unless prevented by clouds. 

 Subject to the remarks a, b, c, d, the times to be recorded near 

 the internal contacts are as follows : — 



At Ingress. — " The time of the last appearance of any well- 

 marked and persistent discontinuity in the illumination of the 

 apparent limb of the sun near the point of contact." 



At Egress. — "The time of the first appearance of any well- 

 marked and persistent discontinuity in the illumination of the 

 apparent limb of the sun near the | oint of contact." 



(a) The expression " soil -marked and persistent discot tinuity 



