36 THE LAW OF THE SQUARES OF THE DISTANCES. 



CHAPTER IV. 



THE LAW OF THE SQUARES OF THE DISTANCES. 



I MUST admit that the title of this chapter is not, at the 

 first view, at all an attractive one. It sounds very mathe- 

 matical. But then there is an interest and a beauty in a 

 mathematical principle when it is once understood, and as 

 this one, known as the Law of the Squares of the Distances, 

 can "be easily understood when properly explained, and as 

 it is one of fundamental importance, not only in its appli- 

 cation to the subject of light, but in countless other cases 

 where we may observe its operations in the phenomena 

 of nature, I hope that none of the more intelligent and 

 thoughtful of the readers of this book will be alarmed at 

 the mathematical aspect of its name. 



The circumstances under which John's attention was 

 first called to it were somewhat curious. He and his 

 cousin Lawrence had been making an excursion that day 

 to the Tower of London, a famous old structure, which was 

 used in former times as a fortress to defend the city from 

 hostile vessels coming up the river. Of course, since this 

 was its object, it was below the city at the time when it 

 was built, but the city has now extended far below the 

 spot on which it stands. I: is, for other reasons also, now 

 useless for any purposes of defense, but it is still preserved, 

 and is used as a museum of curiosities, and contains vast 

 collections of ancient arms and armor, and of a great many 

 other relics of old times which are very curious to see. 



Lawrence and John had been to visit it that day, and 

 had stopped on their return to their lodgings to dine at a 

 coffee-house ; for, as it was uncertain what time they would 



