108 LIGHT SCIENCE FOR LEISURE HOURS. 



thing existing close by the sun's globe can be recog- 

 nised. 



But then, unfortunately, no total eclipse can present 

 these desirable features. If a total eclipse is to be 

 worth seeing at all, the moon's disc as seen at the time 

 must be appreciably larger than the sun's. When 

 totality begins the outlines of the two discs just touch 

 at a single point, and when totality ends the two discs 

 just touch at another point ; but during all the rest of 

 the totality the two outlines do not touch at all, that 

 of the moon surrounding without touching that of the 

 sun. The outlines of the two discs do twice touch, 

 however, in each case for one moment and at one point. 

 What Professor Young determined to do, therefore, was 

 to Bring under special examination that one point 

 where the outlines touch at the exact moment when 

 totality begins. In other words, he directed his special 

 attention to the point where the last trace of the sun's 

 disc was about to disappear. It is perhaps scarcely 

 necessary to say that he did not trust to the powers of 

 his telescope, but that he employed a powerful spec- 

 troscope. And further, he did not depend on his own 

 observations alone, but had adjusted a spectroscope for 

 the use of Mr. Pye, an English gentleman residing in 

 the part of Spain where the eclipse-observing parties 

 were stationed, so that that gentleman also might make 

 the required observations. 



In his account, Professor Young does not mention 

 what he expected to see. It is probable that he had 

 in his thoughts the observations of Fr. Secchi, and 



