194 Mr Russell's Description of a Polyphotal Lamp. 



The lamp is polyphotal, or allows several lights or burners 

 to be used simultaneously in the same reflector. The reflec- 

 tor being of single curA'ature, is easily made, and at little ex- 

 pense. 



The construction of the reflector may be easily understood 

 by conceiving a thin sheet of silver, strengthened by being 

 united to a copper-plate, to be bent round in the middle, so as 

 to coincide with a parabola, and form a species of conoid or 

 surface, whose section, at right angles to the plane of the pa- 

 rabola, is rectilineal. 



In the focal transverse axis of this figure, that is in the line 

 joining the foci of the parabolas, at 

 the two ends of this figure, are placed 

 any number of lights, as in the lamp 

 exhibited. 



The figure which is thus formed 

 has this property, that a plane ver- 

 tical section through any point in the 

 transverse focal axis A B, is a para- 

 bola whose focal distance varies with 

 the angle of divergence. The new focus may be found by the 

 formula : /' = /^ cos &, f being the direct focus of the pri- 

 mary parabola, and /' the focus of the parabola inclined at 

 the angle L 



For the divergent rays, therefore, it follows that, so long as 

 the radius of the flame does not exceed the diff^erence between 

 / and /' in the formula, the parallelism of the rays in a verti- 

 cal plane will be perfect for a certain part of the flame. 



I have taken, of course, for granted, as in the case before 

 us, tluit lateral divergence is wanted, and that vertical diver- 

 gence alone would be considered a loss ; or, in other words, 

 that light is wanted to spread forwards, on both sides, but not 

 upwards or downwards. 



The ends of the lamp are formed in the same way as the top 

 and bottom, but this is perhaps unnecessary where horizontal 

 divergence is of no consequence, as they might be plain sur- 

 faces without much loss. 



It is of consequence, in the case of the lamp exhibited, to 

 have the focus of the reflector short ; this would bring the 



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