2i8 OPTICAL PROJECTION 



desired, within wide limits. The heights and other phenomena 

 of fluids in glass tubes are well shown in this way, and large 

 diagrams traced rather thickly upon plates of glass or mica, 

 demonstrate very well. A Sprengel air-pump can be excellently 

 shown in action by this method ; also large siphons, a full- 

 sized pyrometer, and many experiments in conduction of heat 

 which require more length of conductor than readily comes 

 within the lantern field. ' Cartesian divers ' are excellently 

 shown in this way. Often there will be a choice of the two 

 methods. Thus, the equilibrium of liquids in communicating 

 vessels, the principle of the hydrometer as on page 221, and 

 other experiments in hydrostatics, may easily be shown either 

 by apparatus made on a small scale, to come within the field 

 of the condensers ; or with equal facility by larger apparatus 

 and this method of shadow projection. 



I fear that this simple means of producing screen projec- 

 tions will be regarded with some scepticism, if not derision, 

 by those ignorant of it. I can only state that the sharpness 

 and precision of outline thus attainable will astonish those 

 who try it for the first time ; and that in all cases where a 

 silhouette representation of an operation or piece of apparatus 

 is sufficient, as it is in a vast number of experiments, it is 

 perfectly satisfactory, and unequalled in its convenience for 

 dealing with apparatus generally, of the usual size, as it 

 comes to hand in most physical laboratories. 



110. Intermediate Projections. There is still another 

 expedient available. It may be desirable to produce a really 

 focussed image of some experiment or piece of apparatus, 

 rather than use the shadow method ; and yet the lantern 

 condenser may be too small. If in such a case we have at 

 hand a lens six or seven inches diameter, and this gives 

 sufficient field, 1 we can adopt the arrangement shown in 



1 I have never tested this method with a lens of larger diameter than that. 

 It might possibly be carried farther, only there would be an obvious tendency 

 for the field to be less illuminated at the margin than in the centre. A seven- 



