20 LIGHT SCIENCE FOR LEISURE HOURS. 



observed to great advantage. For she is then seen as 

 a round black spot on the sun s face ; this face is thus 

 as a sort of dial-plate on which the black disc of Venus is 

 as an index. The sharply-defined edge of this black 

 disc presents the same advantage which a neatly-cut 

 index possesses, enabling the observer to measure satis 

 factorily the place of the planet. All the circumstances 

 are favourable, except two: first, the index, that 

 is, the black disc, is not even for an instant at rest ; 

 and, secondly, the index-plate, that is, the sun s disc, 

 is itself displaced by any difference in the position of 

 the terrestrial observers. 



Nothing can be done to remedy the latter circum 

 stance. Its effects are easily seen. Suppose an observer 

 at some northern station sees Venus in reality depressed 

 by a third of a minute of arc, which is about the hun 

 dredth part of the sun s apparent diameter. Then the 

 sun, being farther away in the proportion of about 

 ten to three, is depressed by about the tenth of a 

 minute. Accordingly, Venus only seems to be depressed 

 by the difference of these amounts, or by little more 

 than a quarter of a minute. Nevertheless it is far 

 easier to measure this reduced displacement on the 

 sun s face, than to measure the larger displacement 

 without his face as an index-plate. 



The other circumstance has been dealt with in two 

 ways. 



First, in accordance with a suggestion of Halley s, 

 instead of attempting to measure the position of Venus 

 on the sun s face, the astronomer may simply time her 



