LOCAI; PARTICULAES OF THE TRANSIT OF VENUS, 1874. 83 



the estimated diameter of the drop, the state of the sun parlicu- 

 larly, whether steady, boiling, or whatever it may be, also the 

 magnifying power used, whether Veniis has a well-defined outline 

 away from the drop, in fact everything required to convey to a 

 stranger all the circumstances — atmospheric, telescopic, and per- 

 sonal, then existing. 



After apparent internal contact the drop will very rapidly 

 decrease in size, and in the course of fifteen or twenty seconds it 

 will become a line (No. 5), and suddenly break. This is real 

 internal contact and the last moment before the breaking of the 

 line should be carefully represented on card 4, and another 

 card should be made to show the position of Venus the moment 

 after the line is broken. This is certain contact, the second 

 phenomenon to be observed. 



True internal contact simply means, as will be seen by reference 

 to figure 7, that the true limb of Venus has crossed the true 

 limb of the sun, and therefore allows the light to appear all 

 round it ; but since the motion of Venus on the sun is very 

 slow, the formation of the line of light is gradual, and will 

 be seen sooner by some persons than by others ; and if telescopes 

 of large apertures are used without being stopped down, a 

 considerable difference of time may be made, amounting, 

 perhaps, to ten seconds. But if the sun's limb is unsteady, 

 it becomes almost impossible to catch this phase at a,ll, and 

 m. many cases it will probably not be seen ; though, when 

 caught, it is by far the most satisfactory observation that can be 

 made. It must be remembered that at real internal contact, or 

 the breaking of the drop, Venus will appear some distance 

 within the outline of the sun ; and the apparent amount, as a 

 fraction of the diameter of the planet, should be carefully esti- 

 mated and recorded on the card, as it will aff'ord the means of 

 ascertaining how the observer caught the phase which was 

 required to be observed. 



After this phase has been observed, the direct method must be 

 applied until Venus approaches the other limb of the sun, when 

 the phenomena described at ingress will be reproduced in reverse 

 order. If the formation of the line No. 5, fig 7 can be well observed, 

 it is to be done with all the care of the previous observation, and 

 Venus will arrive at phase 4 in about eighteen seconds afterwards, 

 and the other phases in order — 3, 2, 1. Cards should be used at 

 all phases of egress, and with the same care as at ingress, care 

 being taken to mark each card Egress. The cards I have repre- 

 sented in figures 2, 3, and 4 are drawn to scale, and represent 

 the sun and Venus seen upon it, each of their relative dimen- 

 sions : if thought desirable, cards for intermediate phases may 

 be prepared. If these cards are held in the left hand, by taking 

 hold of the side marked W, then the card represents the part of 



