50 CANDLES TOO TALL. 



two would, of course, give him as strong a light on his pa* 

 per as sixteen candles at the ordinary height would have 

 done. 



Lawrence talked with John about the law of the squares 

 of the distances for some time, showing him that it applied 

 to all cases of the emanation of any force from a centre, 

 such, for example, as heat and gravitation ; and this for the 

 simple reason that the force, or influence, whatever it might 

 be, in receding from the centre, was expanded in two di- 

 mensions, length and breadth, and so the surface within 

 which any given portion of it was included was enlarged 

 in two dimensions, which caused the surface to increase 

 not simply as the distance, but as the square of the dis- 

 tance. And as the intensity of the influence would be di- 

 minished just in proportion as it was diffused over a great- 

 er space, the intensity that is, the force at any one point, 

 would be inversely as the square of the distance. 



This principle of the very great difference in the bright- 

 ness of the light at different distances from the source of 

 it a difference far greater than one, without understand- 

 ing the principle, would suppose is of great importance 

 for all who have to do any work by artificial light, as, in 

 many cases, by diminishing their distance from the light, 

 they can gain a much greater advantage than they would 

 at first imagine. 



There is another principle, also, which it is very impor- 

 tant to understand, and that is the illumination of the pa- 

 per, or the page, or whatever else it is that the light shines 

 upon, depends not merely upon the distance, but also upou 

 the angle at which the rays fall. 



This will be plainly seen by the engraving on the opposite 

 page, which shows that when the same book is held oblique- 

 ly, as it is at "the left, it receives but half as much light as 

 when it is at right angles to the rays, as shown on the right. 



