142 Mr Swan's Formulae for constructing 



Therm. Corrected.^ 



Bocklet (Four miles from Kissingen, Chalybeate.) 



July 1, 4 P.M. 50-7 Troughton. 60'2 



Kapdte (Chapel at Kissingen, fine fresh-water spring in front of, 

 accompanied by much gas.) 



June 28, 6 p.m. 61-5 A 3. 



The above agree usually within a few tenths of a degree with the 

 observations made fully a month later in 1838. 



Formulae for constructing Mr Thomas Stevenson^ s Totality. 

 Reflecting Hemispherical Mirror. By William SwAN, 

 r.Il.S.E., F.RS.S.A. Communicated by the Royal Scot- 

 tish Society of Arts.* 



Among the various arrangements of optical apparatus 

 hitherto used in lighthouses, there is none that possesses the 

 property of collecting all the light which emanates from 

 a flame, and transmitting it in a single beam of parallel 

 rays. The most perfect of these arrangements, which were 

 invented by Fresnel, were not intended to produce this effect ; 

 and the ordinary parabolic reflector, which at first sight 

 might seem fitted to transmit all the rays in one direction, 

 in reality fulfils this condition most imperfectly, as it leaves a 

 large proportion of them to radiate in their natural directions. 



The importance of the problem of transmitting all the 

 light in a single direction will at once be seen, when it is 

 considered that there are only two ways in which it is 

 possible to increase the apparent brightness, and, therefore, 

 the efficiency of a sea-light. One is by increasing the in- 

 tensity of the source of light, the other, by using to most 

 advantage the light actually produced. Now Carcel's me- 

 chanical lamp, as adapted by Fresnel to lighthouse use, is an 

 instrument so perfect, and produces so brilliant a flame, that 

 we shall not probably soon have a better source of light than 

 that which we already possess. The only available method 

 for increasing the efficiency of our sea-lights, is, therefore, to 

 obtain the maximum efi^ect of the mechanical lamp, by trans- 

 mitting all its rays in a single direction. 



* Read before the Society, 25th November 1850. 



