PROJECTIONS. 39 



second plain mirror at r, which should have such an 

 inclination as to reflect the converging rays upon the 

 object to be shown at d^ and strongly illuminate it ; the 

 objective at o will be used in the same way as for any 

 other projection. This apparatus should be in a box 

 made with sides a foot square and six or eight inches 

 deep. At the back of it a hole should be left at dy in 

 which the various objects for exhibition may be held. 



In place of the condenser and the plain mirror, a 

 large concave reflector, such as is used behind lamps, 

 may be placed at r, and the parallel rays from the porte 

 lumiere allowed to fall upon it. It should be placed at 

 such a distance from the object d, that it will just illu- 

 minate it ; this will of course be determined by the 

 focal length of the mirror. 



The room needs to be quite dark for the successful 

 working of this apparatus, and especial care should be 

 taken to prevent any of the light from the porte lumiere 

 from being scattered into the room ; paint the box black, 

 inside and out, with lampblack mixed in japan varnish. 



If the lime light be used, as it generally is for such 

 an exhibition, it is necessary to modify the lantern very 

 much, — so much so as to require an entirely new in- 

 strument. The following is the simplest plan of one : 

 A square wooden box made eighteen or twenty inches 

 on a side, and about fifteen inches deep, may have a 

 little way made in it on one side for the fixtures holding 

 the jet / and the lime j to slide upon. A hole r cut 

 six inches square, may be made near the corner, and 

 another one on the front side for the light to come 

 through upon the lens <?, which is the only lens needed 

 for work. The size of this hole should be no greater 

 than that of the lens o used for the projections, but this 

 lens should be as large as possible. A lens six or eight 



