328 



SECTION MECHANICS. 



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Figure 13. 



B, Fresnel's fixed light: i, 2, 3, 10, 11, 12, 13, 14, straight condensing prisms ; 

 4> 5> 6, 7, 8, 9, new straight twin prisms. 



I have nothing more to add, except with regard to the characteristics 

 of different lights, which I may mention in two words. Formerly there 

 used to be the fixed light and the revolving and coloured lights. But we 

 have now two other characteristics, namely, the flashing light, which is a 

 light producing extremely quick flashes, very distinctive from the revolv- 

 ing light, and the intermittent light which consists of occultations pro- 

 duced by frames coming down in front of the apparatus, and so cutting 

 off the light suddenly, as opposed to the gradual waxing and waning 

 which you have with a revolving light. These lights were introduced 

 by the late Mr. R. Stevenson in 1827 and 1830. In 1871, Mr. R. L. 

 Stevenson proposed an optically perfect intermittent light in which 

 the periods could be made of unequal duration by means of eclipsing 

 mirrors moving horizontally within the glasswork of a fixed light 

 apparatus. 



The PRESIDENT : I will now call upon you to pass a vote of thanks 

 to Mr. Stevenson for his very interesting communication. I have 



