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THE POPULAR SCIENCE MONTHLY 



cle (Fig. 3), the shorter one being her track to the southern observer, the 

 longer to the northern. In Fig. 4, a b is her apparent path in the first 

 case, c d in the second. This figure shows the direction of the planet's 

 motion, and, with approximate truth, its apparent size as compared with 

 the sun and the decree of actual displacement. Its first appearance, 

 touching the outside of the sun as at a, is what is called " first external 

 contact." This is shortly followed by " first internal contact," when 

 the planet has moved wholly on to the sun's face, and is just quitting 

 the edo-e. After some four hours it touches the edge again (" second in- 

 ternal contact"), crosses it and disappears ("second external contact"). 

 The external contacts have not hitherto been much relied on, but, now 

 that with the spectroscope we can see the planet a little way off the 

 sun, they can be better observed. The internal contacts are the im- 

 portant ones, and these have heretofore been rendered more or less 

 uncertain, by the phenomenon called the "black drop," already referred 

 to, as consisting in an optical illusion, by which the planet seems to 

 cling to the limb and pull out of shape, like a drop of ink just about 

 falling from the pen. (Fig. 5.) 





Fig. 5. A Portion of the Sun at First Internal Contact of Venus, showing the 



" Black-Drop." 



Since there is no actual track left to reckon the distance between 

 the chords from, the northern and southern observers time the planet 

 across, very accurately, and, from the times, the lengths of these chords, 

 and hence the distance between them, may plainly be found, since we 

 know just how long the planet would take to go over the sun's diam- 

 eter. There is another way, by measuring the distance, from the sun's 

 centre, of Venus at different stages of her progress, as seen by a pair 

 or any number of pairs of observers ; but probably best of all is pho- 

 tography, which is to be used by nearly every station, and which will 

 give us almost any number of pictures (as many as 150 or 200 to a 

 station), showing exactly how the planet looked from minute to min- 

 ute to the photographer's lens an observer which does not get flur- 



