270 



NATURE 



\Jtily 20, 1882 



in the illumination of the apparent limb of the sun near the 

 point of contact " is intended to guard observers against giving 

 times for the contacts when there may be a suspicion only of 

 some slight disturbance, haze, shadow, or interference pheno- 

 mena. It is a point of primary importance that all the observers 

 shall, as far as possible, observe the same kind of contact ; and 

 it is therefore desirable that the times recorded for contacts 

 should refer to some marked discontinuity in the illumination 

 of the sun's limb, about which there cannot be a doubt, and 

 which may be supposed to be recognisable by all the 

 observers. 



(i. ) If a pure geometrical contact, " contact geometrique sans 

 deformation," is alone seen, this is the only time which can 

 be given for the contact ; but if haze, shadow, ligament, or 

 black drop is seen, then the last time, when any marked 

 discontinuity ia the illumination of the sun's limb near the 

 point of contact is distinctly recognised as independent of mere 

 atmospheric tremor, is a time which should be recorded at 

 ingress ; and the first time at which such a marked discontinuity 

 in the illumination is certain, is a time which should be recorded 

 at egress. But if the haze, shadow, ligament, or black drop is 

 ever seen as dark, or nearly as dark, as the outer edge of the 

 planet, the time of greatest blackness, when it is last seen at 

 ingress, or first seen at egress, as dark or nearly as dark as 

 the outer edge of the planet, is also to be most carefully 

 recorded ; this phase appears to correspond most nearly to 

 « hat is given by some observers as "contact geometrique sans 

 deformation," and probably differs but little from what most 

 observers would call "contact," if restricted to a single phase. 

 Near the time of contact the attention of the observer should 

 be directed to the parts of the sun's limb near the point of 

 contact. The discontinuity of the illumiuation of the sun's 

 limb near the point of contact will be recognised by the contrast 

 between the illumination at and on cock side of the point of 

 contact. 



(c) At ingress the contact can also l.e regarded as the time 

 "when light is about to glimmer all across the dark space 

 between the cusps." For so long as the sunlight has not 

 "glimmered" across the dark spuce between the cusps, there 

 must be " some well-marked and persistent discontinuity in the 

 illumination of the sun's limb near the point of contact." In 

 this definition the attention of the observer is directed to the 

 light of the cusps which is encroaching upon the " dark space " 

 between them, whilst in the definition adopted in these instruc- 

 tions his attention is directed to the disappearance of the dark 

 space between the cusps which is being encroached upon by the 

 light of the cusps. Great care is, however, required when the 

 contact is thus regarded, that the glimmeiing of the light of the 

 "aureole," "penumbra," or "sunlight refracted through the 

 atmosphere of Venus," across the dark space between the cusps 

 be not taken for the contact. The time thus recorded would be 

 earlier than the contact required by about a minute of time. On 

 the other hand, a time must not be given "when sunlight is dis- 

 tinctly recognised" between the limbs without any direct 

 reference t) the time when the dark space between the cusps was 

 last recognised. 



(</) The observer should clearly and distinctly indicate the 

 times which, in his opinion, correspond most nearly to the con- 

 tacts as defined above. But in cases where he has any doubt 

 about the second of time which ought to be given, on account of 

 the gradual obliteration and restoration of the illuminaiion of 

 the sun's limb near the point of contact, or on account of any 

 change in the degree of darkness in the haze seen, he may give 

 two times, with a clear intimation of his inability to say which 

 of the two corresponds most nearly to the time of contact ; and 

 such observations, if the limits of uncertainty aie confined 

 within a few second-, will be amongst the most satisfactory 

 observations which can be made. The observer must not be 

 discouraged from giving the nearest second possible on account 

 of the lingering character of the contact. The change in the 

 angular separation of the limbs of Venus and the sun is only 

 a tenth of a second of arc in about two seconds of lime. It is 

 with seconds of arc, and not seconds of time, that we are 

 ultimately concerned ; and one tenth of a second of arc is a 

 very small quantity to be measured on the sun's limb. In the 

 Transit of 1S74, when, however, the change in the angular 

 separation was much slower than in 1SS2, many observers were 

 discouraged and disappointed at the degree of accuracy attain- 

 able, and the observations appear to have suffered from a feeling 

 on the part of the observers that such observations as they could 



make were worthless. This feeling should be carefully guarded 

 against. 



11. It is hoped that all the observers may be able to observe 

 contacts as defined ; but should an observer see a contact which, 

 in his opinion, does not agree with the definition, he must 

 record the time of contact, and describe the nature of the contact 

 observed, with drawings to illustrate his meaning. It is desirable 

 that an observer should record the times at which any very 

 distinctive phenomena are first or last seen near the contacts. 

 But the multiplication of unnecessary time records near the 

 contacts is, in itself, a serious evil, and should be carefully 

 guarded against ; and, more particularly, records of time corre- 

 sponding to " clear sunlight between the limbs of Venus and the 

 Sun " are to be avoided after all touch, as shown by some recog- 

 nisable disturbance of the illumination of the sun's limb near 

 the point of contact, has ceased at Ingress between the limbs of 

 Venus and the sun. When this touch has once ceased, all sub- 

 sequent records of time, unless accompanied by direct measure- 

 ments of the angular separation of the limbs at these times, 

 afford no possible means of determining the angular separation 

 of the centres of the sun and Venus. Such records are, there- 

 fore, of no direct value ; but unless great care is taken they may 

 be accepted as referring to contacts, and may thus lead to most 

 serious error. 



Attention to this point is more particularly necessary when 

 observations of the contacts are picked up through clouds. If, 

 therefore, times are recorded at "ingress" for "distinct band of 

 light between the edge of Venus and the sun's limb" or "Venus 

 well on the sun's disc," the observer must most distinctly state 

 whether this time record is intended to mean that the haze, 

 :hadow, ligament, or black-drop was certainly seen within a few 

 seconds, at most five, of the recorded time, or whether it is 

 ii. tended merely to state as an isolated fact that the contact was 

 over at the time recorded. Time records of the first class are 

 valuable, but those of the latter class are useless, and may be 

 misleading. 



12. If the limbs of Venus, at internal contact, as defined in 

 10, fall within the sun's disc, then the observer should give, as 

 accurately as he is able, probably to seven or eight seconds, the 

 time at which the limbs of Venus and those of the sun mentally 

 completed would appear to touch. This observation must be a 

 rough one ; but it is desirable in the case indicated to give it as 

 a check upon the principal phase observed. 



13. External contacts should be observed. The value of 

 the exe.nal and apparent coi. tacts, which are referred to the 

 "visible" or "apparent" limb of the sun, will greatly depend 

 upon the extent to which uniformity in the instrumental equip- 

 ments, mid in the brightness of the fields of view, may have 

 been secured. 



14. It is desirable that all observers who have double-image 

 micromet;rs should measure the cusps at egress, and the distances 

 between the limbs of the sun and Venus after internal contact at 

 ingress ; but the eye-pieces shoidd not be changed at ingress 

 until there is a broad band of sunlight between the limbs of 

 Venus and the sun. If an observer feels perfectly confident in 

 his ability to change the double-image micrometer for an ordi- 

 nary eye-piece and to focus properly, after making cusp measures, 

 before internal contact at ingress, then such an observer may 

 venture to make cusp measures at ingress; but it is most 

 earnestly hoped that observers will not run any risk of losing 

 the internal contact observations at ingress for the sake of these 

 cusp measures. It is necessary, not only for the eye-pieces to 

 be changed and the focus found, but that time should be allowed 

 for the eye to accommodate itself to the new eye-piece before 

 the internal contact takes place, or satisfactory observations of 

 internal contact will not be made. 



15. In all cases the recorded times should be those taken 

 directly from the chronometer or clock used. 



16. The maker's name and number of the chronometer or 

 clock used should be given. 



17. The errors of the chronometers and clocks should be 

 given for a few days before and after the transit, and a clear 

 statement made of how these errors have been determined. 

 Chronometeis should be compared, whenever possible, [before 

 and after the transit with some standard clock. 



iS. The greatest care should be taken to insure the accuracy 

 of the entries of times from the clocks and chronometers used 

 in the contact observations, and unusual care is required in the 

 verification of the minutes and the half-minutes. In most 

 astronomical observations, if the seconds are recorded correctly, 



