108 LIGHT SCIENCE FOR LEISURE HOURS. 



sufficed exactly to fill up the gaps caused by the 

 absorptive action of those vapours. Secchi reasoned in 

 this way : If we examine the intensely bright light of 

 the sun shining through these vapours, we see that 

 this light is deprived of certain rays, and so dark lines 

 appear ; but if we could examine the light of the 

 vapours themselves we should see that this light is 

 composed of these self-same rays, and so bright lines 

 on a dark background would appear. Now the latter 

 we cannot do on account of the extreme shallowness of 

 the complex atmosphere ; we can, however, by examin- 

 ing the very edge of the sun, obtain light so combining 

 the two kinds that there will neither be dark lines on 

 a bright background nor bright lines on a dark back- 

 ground, but a continuous rainbow-tinted streak pro- 

 duced by the combination of the two. 



It seemed to Professor Young, of America, that 

 during the eclipse something more might be achieved. 

 For at the very moment when the moon has just 

 concealed the true solar disc, the light of the shallow 

 atmosphere must be shining alone. The reader can 

 readily illustrate the case as follows : Let a shilling 

 represent the sun's disc, the narrow raised edge 

 representing the shallow solar atmosphere ; and let a 

 halfpenny be passed over the shilling, as the moon's 

 disc passes over the sun's in an eclipse. Then as soon 

 as the advancing edge of the halfpenny has reached 

 the inner boundary of the raised edge, it will be seen 

 that a fine sickle of that raised portion remains in 

 view, the thickest part of the sickle being at the point 



