for the purposes of Illumination in Lighthouses. 63 



the vertical ones which remain, yet in lighthouses, 

 they are of less use, as the width of the column, in a 

 vertical plane, is necessary to embrace a wider extent 

 of sea. 



4. From the very mode of fitting up the lens AB B'A', it is 

 obvious that we can give it a much greater diameter in 

 a vertical direction, and at less expence, than could be 

 done while it has either a square or a circular form. 



In lighthouses where it may be convenient to employ the 

 reflectors, and Argand burners of the old system, the following 

 arrangement of them with lenses will be found to constitute 

 a cheap and effective apparatus for distinguishing lights. In 

 Fig. 3. AB, AC, A'B', A'C', are the sections of two truncated 

 polyzonal lenses, the elevation of which is shewn in Fig. 4. Ar- 

 gand burners F, F', are placed in the foci of the four lenses, 

 and each of the two burners is surrounded with a parabolic re- 

 flector Pmn' Q, P'ojt/Q', having openings mn, m' ri, op, o' p', 

 sufficiently large to afford a passage for the cones of rays to the 

 lenses AB, AC ; A'B', A'C'. By this arrangement we shall 

 have eight beams of light, namely two powerful columns BRRC, 

 B'R'R'C', produced by the lenses, two columns PQRR, 

 P'Q'R'R', produced by the reflectors, and four of much inferior 

 intensity ABrr, AC r r, A'BW, A'C'r'r', produced by the 

 oblique passage of the cones F'AC, F'A'C', FA'B', FAB, 

 through the lenses. These last columns, will have a slight 

 convergency, 'as the burners which produce them are placed 

 a little without the principal focus of the lenses ; but this evil 

 may be remedied, by bringing the burners F, F' as near as pos- 

 sible, and placing the lenses AB, AC, A'B', A'C', at an angle. 

 The effect of this will be to divide the columns BCRR, 

 B'C'R'R', into four, so that we shall thus have ten columns of 



