LIGHTHOUSE ILLUMINATION. 203 



undertaken, and here is .the place to describe Fresnel's 

 original models and apparatus which are here before you. 

 I should mention here a little discrepancy of terms in 

 various authors. In general, reflecting lights are called 

 catoptric ; such as are not reflected but are refracted, where 

 lenses are used, are usually called dioptric lights. Lastly, 

 what I shall presently speak of, certain rings separate from 

 the interior lens, are called catadioptric rings. Mr. Steven- 

 son has introduced into one of his books the term catadiop- 

 tric for a combination of reflection and refraction. 



Up to Fresnel's time paraboloidal reflectors were em- 

 ployed, consisting in a paraboloidal mirror of polished 

 metal (generally silvered copper) or composed of facets of 

 glass as in this model. At that time it was extremely 

 hard to obtain glass of the requisite curvature, and they 

 were composed of these small facets. There is a great 

 advantage in having glass over metal because there is 

 a great deal more of the light reflected, and not lost. A 

 lamp is placed at the focus of the mirror, and its rays 

 striking the mirror are reflected horizontally outwards. 



The loss of light from highly polished reflectors is con- 

 siderable, varying from '31 to '35 of the entire beam, when 

 the whole amount is taken as unity, and from lighthouse 

 reflectors which cannot be kept at a very high polish it is 

 even greater, being about '444, or very nearly half. But 

 besides this it will be seen that a large proportion of the 

 rays from the lamp are cast upwards and downwards be- 

 yond the limits of the reflector, and are thus completely 

 useless ; but even with this loss this reflector gave so much 

 light that it was used for a great many years, and is used 

 st:ll. If a number of such reflectors and lamps are placed 

 side by side, either on a polygonal or circular framing, and 

 several tiers, placed one above another, a bright light is 

 produced. The polygonal form is for a revolving light, the 

 circular for a fixed one. This system is still employed in 

 many lighthouses, and such reflecting apparatus are called 

 catoptric. 



In his dioptric apparatus Fresnel placed round one 

 single central lamp a series of planoconvex glass lenses 

 (such as you see in the central portion or drum of this 

 model), which, by their well-known property, refracted 

 horizontally the rays of light from the lamp, and in order 



