LIGHTHOUSE ILLUMINATION. 211 



so as to give a certain amount of divergence (without in- 

 terfering with the holophote for the central cone of rays). 

 He recommends mirrors of silvered glass fixed together 

 by Canada balsam, a substance which has nearly the index 

 of refraction of plate glass. I have now given a very brief 

 description of these beautiful apparatus, and if I had time 

 I should wish to give some description of the system em- 

 ployed for insuring a steady burner for the lamp, but I 

 believe you will hear this portion of the system described 

 by one much better skilled in the subject than I am. I 

 will, however, just direct your attention to the concentric 

 wicks employed for burning oil ; the transparency of flame 

 rendering the internal portion available. The mechanism 

 for keeping up the supply of oil is an ordinary clockwork. 

 Now the electric light is often employed in place of rock 

 oil and paraffin. In all cases when necessary it is possible 

 to vary the colour of the light by colouring the lenses and 

 prisms in the portions required. Red is the ordinary 

 colour used when a second colour is required, but other 

 colours may be useful for purposes of distinction. 



On the table are examples of lamps for ship's sides of 

 various colours on Fresnel's principle. 



