72 LIGHT SCIENCE FOR LEISURE HOURS. 



to say nothing of its being measurable ? Yet it is just 

 sucli a problem as the measurement of that line which 

 the astronomer would have to solve. 



But even this is not all. In our illustration only 

 one observer is concerned, and he would be able to use 

 one set of instruments. Suppose, however, that from 

 one end of the two-feet line an observer using one set 

 of instruments took the bearing of the steeple; and 

 that, half a year after, another observer brought an- 

 other set of instruments and took the bearing of the 

 steeple from the other end of the two-feet line, is it not 

 obvious how enormously the uncertainty of the result 

 w r ould be increased by such an arrangement as this ? 

 One observer would have his own peculiar powers of 

 observation, his own peculiar weaknesses; the other 

 would have different peculiarities. One set of instru- 

 ments would be characterized by its own faults or 

 merits, so would the other. One series of observa- 

 tions would be made in summer, with all the dis- 

 turbing effects due to heat ; the other would be made 

 in winter, with all the disturbing effects due to 

 cold. 



The observation of the sun is characterized by all 

 these difficulties. Limited to the base-lines he can 

 measure on earth, the astronomer must set one ob- 

 server in one hemisphere, another in the other. Each 

 observer must have his own set of instruments ; and 

 every observation which one has made in summer will 



