Mr Russell's Description of a Polyphotal Lamp. 195 



burners too close to the back, and as light is required for dis- 

 persion in the vicinity, the back is cylindrical, and concentric 

 with the transverse focal axis A B. The effect of t?iis is seen 

 in the annexed diagram, where F is the focus of the parabola 



and the centre of the attached semicircle, whose radius is equal 

 to the semiparameter. 



The use of such a lamp in a steam-vessel, is to shew an 

 object a-head of tiie vessel, and to assist them in navigating 

 narrow channels or rivers at night. The canal-boats go now 

 with these lamps, in a dark night, at the rate of ten miles an 

 hour, as safely as in the day. 



The same principle is applicable to a pier-light, or a direct- 

 ing light, wliich extends throiigh a part only of the horizon, 

 so as to render lenses inapplicable. This arrangement is 

 shewn in the following drawing : — 



'N^- 



-a.— 



The lines /R, /R, &c., give the rays divergent in every 

 direction horizontally, and only rendered parallel in the verti- 

 cal planes, through eacli of the foci ./'//. The zone of light 



