1901-2 TRANSACTIONS. 55 



The flashes are produced by revolving an arrangement of 

 lenses around the lamp as a centre, and much ingenuity has 

 been displayed in the' details for carr^ang round the heavy 

 machinery with the least possible friction. Today ball bear- 

 ings are used on small apparatus, while the largest and most 

 quickly revolving apparatus are floated in troughs of mercury. 

 The most perfect development theoretically of this system of 

 concentration of the rays is the French invention know as the 

 feu kclair. This light is based on the laws that the time 

 which a flash takes to make an impression varies inversely as 

 its intensity, and that once having made an impression a cer- 

 tain time is necessary to allow a steady light to produce its 

 full effect, and that this time also varies inversely as the inten- 

 sity of the light. It has been demonstrated that a very power- 

 full beam will produce its full effect on the eye in the space of 

 i-io second. Applied to lighthouses, this means that there is 

 no advantage in making a flash of longer duration than i-io 

 second, for this will allow it to be seen as a flash at the utmost 

 distance at which it can possibly affect the eye, while as it is 

 approached it will of course appear longer and stronger. 



In the feux eclairs the lenses are so designed as to gather 

 the whole of tlie light into one narrow beam, and are revolved 

 so rapidly as to give flashes of about i-io second duration at 

 intervals of 10 seconds. The resulting speed of the revolving 

 beam of light is very great. Seventy miles is not an extreme 

 limit of visibility for the most powerful lights, and at that 

 radius the beam is travelling at the rate of about 440 miles in 

 10 seconds. Its width, therefore, to act on the eye, for i-io 

 second, must be nearly 4 5^ miles. To give the necessary 

 divergence to the beam, an extremely large size of flame is 

 required' and in practice it js found impossible to maintain a 

 flame large enough to give extreme results. The sailor, who 

 usually cares nothing about theories, does not seem to take 

 kindly to the fett eclair. He complains that it is impossible 

 to locate a flash before it disappears, and he prefers a light of 

 which he can take a bearing- while it remains visible. There- 



