24i4i W. M. Campbell — JSTotes on the Transit of Vemcs. [Dec. 



To guard against this uncertainty, every one has agreed to observe — 



At Ingress. The first distinct lessening of the intensity of shadow- 

 between the limbs. 



At Egress. The first definite appearance of shadow as intense as the 

 disc of Venus. 



After the first there may still remain a shadow less dark than Venus, 

 which will die away gradually ; while on the other hand, at Egress, such a 

 shadow may precede the fully black shade. 



The two phases above described are the phenomena to be observed, 

 but in addition to them, if it can be done without imperilling their observa- 

 tion, the last and first appearance of shade of any kind must be noted. 



The times may also be noted at which it is estimated that the two 

 limbs would just touch, if clearly seen without any distortion, Venus being 

 just within the sun's limb. 



Lastly, at Egress, an attempt may be made to note the last contact of 

 the limbs, Venus being outside the sun, i. e. the last external contact. 



All observations will be useless unless the time is accurately known, and 

 the comparison of the clock or chronometer used, with a clock whose error 

 and rate are well determined both before and after the observations, should 

 form part of the record of the observations. 



Or, if the chronometer is compared with daily time signals, the com- 

 parison should be continued for one or two days after the Transit and 

 included in the record with the comparison of the preceding days. In any 

 case, the method by which the recorded times are arrived at, should be fully 

 explained, and the observations of all sorts, with full means for testing the 

 whole of the instrumental adjustments. If a transit instrument is used, the 

 Level readings, Collimation tests, &c. and a full description of the instrument 

 must be given. 



The telescope used must be described as well as possible, as regards 

 opening of object-glass, focal length, magnifying power, &c. 



The place of observation must also be accurately described, or its 

 latitude and longitude given. 



Note. — The Transit of Venus having taken place since the above was written it 

 may be interesting to state before going to press that the Transit was successfully ob- 

 served in India, by Col. Tennant's party at Roorkee where 107 six-inch photographs and 

 5 Janssen plates were taken, with favourable weather ; at Lahore by Captain Gr. Strahan 

 B. E. ; at Masuri by Mr. J. B. N. Hennessey, who obtained some interesting results 

 with the spectroscope; at the Surveyor General's OflB.ce, Calcutta, where 39 photographs 

 and several eye observations were made ; at Muddapur by a party of Italian astronomers 

 under the direction of Sig. Tacchini, the distinguished spectroscopist, and at Kurrachce 

 by General Addision. At Madras the weather proved unfavourable. 



Tidings of the observations have also been received from the parties scattered in 

 various parts of the world, mostly satisfactory. 



