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 



