LIGHT 



3425 



LIGHTHOUSE 



The Spectrum. One of the most beautiful 

 of all natural phenomena is the bow of fairy- 

 like colors that spreads across the sky when the 

 sun shines on a mist or a shower of rain. Sir 

 Walter Scott, in Marmion, asks 



What skilful limner e'er would choose 

 To paint the rainbow's varying hues, 

 Unless to mortal it were given 

 To dip his brush in dyes of heaven? 



It was not until late in the seventeenth cen- 

 tury that even scientists understood the cause 

 of the rainbow, at which mankind had won- 

 dered since the day when God "set His bow 

 in the heavens." To Sir Isaac Newton is due 

 the honor of the explanation. He showed that 

 all the colors of the rainbow are the compo- 

 nent parts of white light, and that the bow is 

 caused by the refraction and reflection of the 

 sun's rays when they strike the drops of water 

 at certain angles. 



Fig 5 shows how Newton produced a band 

 of colors similar to those of the rainbow. He 



Mirage 



Mirror 



Polarization of Light 



Rainbow 



Reflection 

 Spectroscope 

 Spectrum Analysis 

 Telescope 



Fis-5 



admitted a beam of sunlight through a small 

 hole in the shutter of a darkened room, and 

 in the path of the beam placed a glass prism 

 and a white screen. The light in passing 

 through such a prism is both refracted and 

 spread out, appearing on the screen as a band 

 of colors arranged in the following order: vio- 

 let, indigo, blue, green, yellow, orange, red. 

 Such a band of colors is called a spectrum. 

 The order of the colors in the spectrum may 

 be remembered by combining their initial let- 

 ters into the "word" vibgyor. C.R.M. 



Related Subjects. The following articles in 

 these volumes will be of interest in connection 

 with a study of light : 



Aberration Ether 



Camera Fluorescence 



Color Gas 



Diffraction Lamp 



Electric Light Lens 



Electromagnetic Limelight 



Theory of Light Microscope 



215 



LIGHTHOUSE. The safeguarding of the 

 mariner from rocks and shoals and other per- 

 ils of the sea, by means of beacon lights, is 

 an old, old story. We may trace the develop- 

 ment of the lighthouse from the period when 

 the ancient Libyans erected towers on the 

 northern coast of Egypt, and hung from pro- 

 jecting poles braziers filled with burning fuel, 

 whose light was a signal of warning to the 

 sailor. Classic writers tell of a lighthouse 

 erected about 300 B. c. on the island of Pharos, 

 known as the Pharos of Alexandria, and famed 

 as one of the seven wonders of the ancient 

 world. The Romans built light towers at sev- 

 eral ports of the empire, and after the conquest 

 of Britain they safeguarded vessels on the 

 stormy English Channel by erecting light- 

 houses at Dover and at Boulogne, on the 

 French coast. The tower at Boulogne was a 

 guide to mariners for over fourteen centuries, 

 and was one of many similar structures that 

 protected the navigators of the Middle Ages. 



Modern Lighthouses. Lighthouses of to-day 

 not only serve to warn vessels of approach to 

 reefs and shoals, but they are also erected at 

 appropriate points to guide mariners into the 

 entrance of harbors. Some are built on iso- 

 lated rocks out at sea, some on bluffs or prom- 

 ontories along coasts, and others have their 

 foundations in shallow parts of the sea at long 

 distances from shore. Their construction is 

 therefore determined by their location and the 

 objects for which they are built. Lighthouses 

 exposed to the fury of fierce ocean gales, the 

 constant beating of wind and wave, or the 

 pressure of floating ice, must be solidly built 

 on sure foundations. The tower of stone ma- 

 sonry, nearly cylindrical in form but gradually 

 narrowing from base to top, is typical of many 

 older lighthouses, while huge, cylindrical towers 

 of steel, bolted into solid rock or constructed 

 on foundations of stone or concrete are repre- 

 sentative of later designs. Stone, brickwork, 

 concrete, structural cast iron, steel and timber 

 have all been employed in lighthouse building. 



On land sites, where there is ample space 

 about the tower, the keeper's quarters, work- 

 shop, storage rooms, etc., are in adjoining 

 buildings, but the various compartments are 

 housed in one structure when the light station 

 is built out at sea. Occupants pass from room 

 to room by means of a winding staircase. The 



