DEMONSTRATIONS <9/< APPARATUS 213 



In the way of limitation, it is plain that, except so far as 

 any piece of apparatus may be transparent or semi-transparent, 

 the projection can only appear as a black shadow, and fluids 

 in tubes, however transparent, will also appear more or less 

 opaque. For a large class of apparatus this matters little, 

 however ; especially as in many cases motion of some kind is 

 the essence of the experiments. Thus, in the electroscope, the 

 effect to be shown upon the screen is simply the divergence of 

 the leaves, which is sufficiently clear. It is further obvious, 

 that the size, of apparatus to be projected in this way is limited 

 by the field of the condensers ; and that, in fact, the possibility 

 of exhibiting a vast number of experiments by the projection 

 method, depends simply upon the possibility of reducing the 

 requisite apparatus to a sufficiently small size. 



The chief difficulties are also two. The first is, that the 

 object to be projected no longer lies in the same focal plane, 

 and can therefore not be all truly in focus at one time. This 

 difficulty is best overcome by using a projecting lens of longer 

 focus than would be used for diagrams alone, 9 to 12 inches 

 focus being most suitable. The objects are coarse compared 

 with the small letters and lines of a diagram, and such a lens 

 gives an amply large scale, while the difference in focus be- 

 tween various parts of the object is much less evident. With 

 such a lens the difficulty about focus is much less observable 

 than might be supposed, and a vast mass of apparatus, which 

 need not be particularised, is perfectly well shown. Let ua 

 take, as a fair illustration, a radiometer. While in the electro- 

 scope the glass case may be neglected, and only the leaves 

 focussed, in the radiometer even the active portion of the 

 apparatus cannot all be in focus together, but the revolving 

 vanes will occupy planes differing by one to two inches. 

 Nevertheless the instrument, and the effect of revolution, will 

 be shown upon the screen perfectly well. 



The other difficulty is greater, and is too often allowed 

 to spoil the projection altogether. It is especially difficult to 



