for the purposes of Illumination in Lighthouses. 61 



This description of distinguishing lights, which we have ta- 

 ken from Mr STEVENSON'S excellent article on Lighthouses, in 

 the Edinburgh Encyclopaedia, indicates very distinctly the defects 

 of the present methods, the great importance of resuming the 

 subject, and the particular points which demand the attention 

 of the scientific inquirer. In the construction of distinguish- 

 ing lights, three methods may be adopted : 



. 



1 . The first method consists in making one or more lights 



disappear and reappear in regular succession, by their 

 revolution round a vertical axis. 



2. The second consists in tinging the columns of light with 



the different colours of the spectrum. 



3. And the third consists in the combination of these two 



methods. 



If the lighting apparatus consists of two large lenses, of 

 which Fig. 2. Plate V. is a section, and if it is made to revolve 

 round a vertical axis thirty times in an hoyr, the brilliant co- 

 lumn of light LRLR will be seen every minute, and it will be 

 preceded and followed by the other columns which surround it. 

 If the large lenses are four in number, the same effect will be 

 produced by a rotation of fifteen times in the hour ; or, by ma- 

 king the velocity of rotation the same as before, the disappear- 

 ance and reappearance of the lights will follow each other with 

 greater rapidity. If a zone of eight equal lenses is used, an 

 eclipse and a brilliant light will be seen eight times during every 

 revolution ; and this may be varied, by making each alternate 

 lens of inferior power, so that there will be a transition to total 

 darkness by two different intensities of brilliancy. 



In constructing a distinguishing light on this principle, I 

 propose that the lenses shall have the form of a parallelogram, 

 and shall be arranged so as to form the faces of an eight-sided 



