36 LIGHT SCIENCE FOR LEISURE HOURS. 



issued a thick octavo volume crowded with such pre- 

 dictions, three or four years in advance of the events 

 predicted ; and these predictions are accepted with as 

 little doubt by astronomers as if they were the records 

 of past events. 



But astronomers are not only able to predict they 

 can also trace back the paths of the celestial bodies, 

 and say : * At such and such a long-past epoch, a given 

 star or planet occupied such and such a position upon 

 the celestial sphere.' But how are they to verify such 

 a statement ? It is clear that, in general, they cannot 

 do so. Those who are able to appreciate (or better, 

 to make use of) the predictions of astronomy, will, 

 indeed, very readily accord a full measure of con- 

 fidence to calculations of past events. They know 

 that astronomy is justly named the most exact of 

 the sciences, and they can see that there is nothing, 

 in the nature of things, to render retrospection more 

 difficult than prevision. But there are hundreds who 

 have no such experience of the exactness of modern 

 astronomical methods who have, on the contrary, a 

 vague notion that modern astronomy is merely the 

 successor of systems now exploded ; perhaps even that 

 it may one day have to make way in its turn for new 

 methods. And if all other men were willing to accept 

 the calculations of astronomers respecting long-past 

 events, astronomers themselves would be less easily 

 satisfied. Long experience has taught them that the 

 detection of error is the most fruitful source of know- 

 ledge ; therefore, wherever such a course is possible, 



