.386 OPTICAL PROJECTION 



mirror of silvered micro-glass may be cemented to the index, 

 and by it a small parallel beam focussed from an aperture 

 reflected to the screen ; the deflection of the spot, with any 

 decently good instrument, will show the effect even of the 

 warmth of the hand. Or a series of small thermostatic com- 

 pound bars is easily constructed, whose action can be pro- 

 jected in the field of the condensers. 



In liquids, any thermometer with a transparent scale can 

 be projected, using the erecting prism. That the expansion 

 is due to greater separation of the particles, and lowers the 

 specific gravity, is easily shown by paraffin oil 

 stained with aniline colour, in a U-tube, as in fig. 210. 

 One leg of the tube is surrounded by a much wider 

 tube fitted on by a cork at the bottom. The liquid 

 in both tubes stands at the same level ; but on 

 filling the large tube with boiling water, the levels 

 being in the field of the condensers, it will be seen 

 that the liquid in the heated leg stands considerably 

 higher than in the other. 



Holding a flask of very thin glass with a very 

 small tube neck, which is filled with coloured liquid, 

 in a larger vessel, and projecting the part showing 

 the level in the tube, on pouring hot water into 

 the outer vessel, the curious effect is produced of 

 a momentary sinking of level, as from & to a 

 (fig. 211) after which the fluid rises as expected, 

 thus showing the expansion of the flask before the 

 liquid itself is heated, and the subsequent superior 

 expansion of the liquid. 



^ ne expansion of air can be projected in any 

 way, but is perhaps most readily shown by pro- 

 viding a flask with a tube of almost capillary bore, and 

 introducing a drop of any coloured fluid when the air is 

 warm, which will be drawn some way down the tube. On 

 projecting this, any difference of temperature will move 



