158 LIGHTHOUSE ILLUMINATION. 



lights by means of fires ; and after that there 

 was introduced a very important refinement in 

 the mode of dealing with the light, namely the 

 principle of reflection ; for understand this 

 (which is not known by all, and not known by 

 many who should know it), that when we take 

 a source of light, a single candle, for instance, 

 giving off any quantity of light, we can by no 

 means increase that light : we can make ar- 

 rangements around and about the light, as you 

 see here, but we can by no means increase the 

 quantity of light. The utmost I can do is to 

 direct the light which the lamp gives me by 

 taking a certain portion of the rays going off on 

 one side and reflecting them on to the course of 

 the rays which issue in the opposite direction. 

 First of all, let us consider how we may gather 

 in the rays of light which pass off from this 

 candle. You will easily see that if I could take 

 the half rays on the one side and could send 

 them by any contrivance over to the other side, 

 I should gain an advantage in light on the side 

 to which I directed them. This is effected in 

 a beautiful manner by the parabolic mirror, by 

 means of which I gather all that portion of the 

 rays which are included in it ; upwards, down- 



