284 ELECTRIC LIGHTING. 



to be projected on a screen in order to be visible to a whole 

 .audience. There are certain of these (relating to the nature 

 of light itself) which require an extremely intense light to 

 show them. No doubt, with solar light and a heliostat, the 

 problem may be immediately and cheaply solved. But 

 more often than not, the sun is absent when he is required, 

 and we are forced to forego those experiments which not 

 only impart greater interest and attraction to a course of 

 lectures, but which are much better understood and much 

 better remembered when they have been strikingly presented 

 to the eye. The electric light can be most successfully sub- 

 stituted for the sun in this kind of application, and Duboscq's 

 regulators have, as we have seen, been arranged purposely 

 for that object. 



The apparatus for projecting the electric light consists of: 

 ist, an arrangement for steadying the light, so that the con- 

 sumption of the two carbons does not displace the luminous 

 point ; 2nd, a closed lantern containing the regulator ; 3rd, 

 a plano-convex lens, for making parallel the divergent rays 

 -coming from the luminous point ; 4th, of a system of optical 

 apparatus, which we cannot here discuss without departing 

 from the subject of this work.* We shall describe only the 

 lantern, as that is a consequence of the electric regulator. 

 . Duboscq's lantern is formed of a kind of bronzed copper 

 box, which surrounds the upper part of the regulator. To 

 economize space, the column of the regulator is enclosed in 

 a sort of chimney in which the box terminates. To hermeti- 

 cally close the box, small shutters, moved by rackwork, close 

 t'he top and bottom of the box at the same time that its 

 door is closed, so that the openings made in the instrument 

 for introducing the regulator are completely shut. The inside 

 of the lantern is provided with a reflecting mirror and two 

 supports, on which two other mirrors can be fitted, in ordei 

 to throw the light on the lenses of a certain apparatus called 



* See my account of the method of projecting the principal phenomena of 

 optics by means of Duboscq's apparatus. 



