LIGHT. 



99 



In the diverging beam from the lens hold a 

 heated poker or any well heated body at a. The 

 air that is heated in its neighborhood will so deflect 

 the rays of light as to make quite a fine appearance 

 of heat upon the screen. An alcohol lamp lighted 

 and held there will have its whole projection upon 

 the screen: the lamp and flame in outline shadow, 

 while the heated gases rising make an interesting 

 picture. 



To present a still more striking case, project a large 

 solar spectrum, as described on another page, and about 

 half-way between the prism and the screen hold the 

 lighted alcohol lamp, moving it slowly along in the 

 different colors of the spectrum. 



4 LENSES. 



The refractive power of different forms of lenses 

 may be shown by holding them in the beam of 

 parallel rays. The refractive power of a lens of 

 water is seen by taking two 

 watch glasses or one watch glass, 

 and a piece of plain glass a little 

 larger, and bringing the two to- 

 gether under the surface of water. 

 The space between these will be 

 quite filled and they will adhere 

 tightly enough for the experi- 

 ment. 



Fig. 75. 



The same thing will also be exhibited by placing a 

 watch glass upon the upper ring of the vertical attach- 



