76 LIGHT SCIENCE FOR LEISURE HOURS. 



wide. And, besides, if this were the true moment of 

 contact, what eye could be trusted to determine the 

 occurrence of a relation so peculiar ? Yet the interval 

 between this phase and the final or peg-top phase 

 lasted several seconds as many as twenty-two in one 

 instance in 1769 and the whole success of the obser- 

 vation depended on exactness within three or four 

 seconds at the outside. 



We know that Venus will act in precisely the same 

 manner in 1874. If we had been induced to hope 

 that improvements in our telescopes would dimmish 

 the peculiarity, the observations of the transit of 

 Mercury in November 1868 would have sufficed to 

 destroy that hope, for even with the all but perfect 

 instruments of the Greenwich Observatory, Mercury 

 assumed the peg-top disguise in the most unpleasing 

 manner. 



It may be asked, then, What do astronomers propose 

 to do in 1874 to prevent Venus from misleading them 

 again as she did in 1769 ? Much has already been 

 done towards this end. Mr. Stone undertook a series 

 of careful researches to determine the law according to 

 which Venus may be expected to behave, or to mis- 

 behave herself; and the result is, that he has been able 

 to tell the observers exactly what they will have to 

 look for, and exactly what it is most important that 

 they should record. In 1769, observers recorded their 

 observations in such doubtful terms, owing to their 

 ignorance of the real significance of the peculiarities 

 they witnessed, that the mathematicians who had to 



