158 OPTICAL PROJECTION 



and other diagrams. A suitable bi-unial, the simplest modi- 

 fication of the ordinary form, is that in fig. 84, and will per- 

 fectly suffice for such purposes, where physical experiments 

 are not required, 



My own bi-unial lantern, being required for experiment, 

 as well as exhibiting slides and microscopic projections, was 

 designed differently. The bottom is supported on four brass 

 pillars, each 4 inches high, which allow gas-pipes for coloured 

 flames, or any other apparatus, to be readily introduced from 

 below, while a black curtain round these pillars, and another 

 round the back of the lantern, stop all stray light when re- 

 quired, the dark blue sight-holes being also covered by brass 

 shades. The top lantern is rarely used for anything but slides 

 and diagrams, and another similar front for the bottom one 

 converts the whole into an ordinary dissolving lantern when 

 required. This lower front is interchangeable with the 

 microscope front ; or both can be removed, leaving the bottom 

 lantern free for parallel-beam work or physical projections. 

 The base-board is also made to slide out, or close up shorter 

 as required ; and although the whole lantern is too small to 

 make regular work advisable with the arc-light, the jets are 

 so removable that the lamp presently described could be used 

 if required. The wooden body is of teak, darkly stained and 

 dull-polished. I believe the instinct which leads a scientific 

 demonstrator to desire something which looks different from 

 the ordinary ' magic-lantern ' of the itinerant lecturer, to be a 

 sound one ; at all events I plead guilty to the weakness, while 

 the dark wood and brass pillars have actual advantages in 

 practice. 



Such a lantern is not, however, suitable for constant work 

 with the arc-light. A very well-known pattern of double arc 

 lantern is Ladd's, better known as the Browning model, shown 

 in fig. 85, where two nozzles, one reserved for diagrams, and 

 the other generally fitted on Duboscq'ssliding-condenser plan, 

 centre on the same arc-light at the centre of the circle. This 



